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 Cβ–H bond of β-O-4
ketone to promote the insertion of molecular oxygen, and subsequently,
Cu/C3N4 catalyzes the selective cleavage of
the Cα–Cβ bond. The catalysts
were also active for the oxidative cleavages of other lignin models
and oxidized organosolv lignin.