Highly efficient oxidative cleavage of lignin β-O-4 linkages via synergistic Co-CoOx/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-CoOx) 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 Cβ–O and Cα–Cβ 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.
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