Hexaniobate as a Recyclable Solid Base Catalyst to Activate C-H Bonds in Lignin Linkage Boosting the Production of Aromatic Monomers

The targeting cleavage of lignin into value-added aromatic monomers has attracted increasing attention. Although the base-catalyzed depolymerization of lignin has been developed, the use of excess corrosive bases and their poor recyclability limit their industrial implementation. Herein, a catalytic amount of solid base K₇HNb₆O₁₉ (KNb₆) coupled with copper-modified graphitic carbon nitride (Cu/C₃N₄) shows enhanced catalytic performance for the oxidative cleavage of β-O-4 lignin linkages using molecular oxygen. Due to the synergetic effect between KNb₆ and Cu/C₃N₄, 96% of the β-O-4 ketone model was converted under relatively mild conditions, and phenol (yield: 90%) and other aromatic monomers (yield: 90%) were obtained. Moreover, KNb₆-Cu/C₃N₄ is robust, and its catalytic activity is basically maintained after five cycles. Experimental and theoretical studies (spectroscopic analysis, control experiments, kinetic study, density functional theory calculations, etc.) reveal that the surface basic O atoms of KNb₆ can simultaneously activate the C_β-H bond of β-O-4 ketone to promote the insertion of molecular oxygen, and subsequently, Cu/C₃N₄ catalyzes the selective cleavage of the C_α-C_β bond. The catalysts were also active for the oxidative cleavages of other lignin models and oxidized organosolv lignin.