Hexaniobate as a Recyclable Solid Base Catalyst to Activate C–H Bonds in Lignin Linkage Boosting the Production of Aromatic Monomers
Jie Li, Zhen Li, Jing Dong, Renbo Fang, Yingnan Chi, Changwen Hu
Abstract
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 K 7 HNb 6 O 19 (KNb 6 ) coupled with copper-modified graphitic carbon nitride (Cu/C 3 N 4 ) shows enhanced catalytic performance for the oxidative cleavage of β-O-4 lignin linkages using molecular oxygen. Due to the synergetic effect between KNb 6 and Cu/C 3 N 4, 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, KNb 6 –Cu/C 3 N 4 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 KNb 6 can simultaneously activate the C β –H bond of β-O-4 ketone to promote the insertion of molecular oxygen, and subsequently, Cu/C 3 N 4 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.