Nitrogen-Doped Carbon-Modified Cobalt-Nanoparticle-Catalyzed Oxidative Cleavage of Lignin beta-O-4 Model Compounds under Mild Conditions

A noble-metal-free Co-based catalyst, derived from pyrolysis of natural vitamin B-12 on activated carbon, is developed for the first time for one-pot oxidative cleavage of lignin linkages to phenols and aromatic esters with molecular oxygen as the oxidant under mild reaction conditions. High yields of phenol were obtained, and no oxidative coupling of phenol was produced based on the present cobalt catalyst. Compared to the previous report, the present catalyst can achieve the oxidative cleavage of beta-O-4 ketones even at room temperature using a dioxygen balloon. The heterogeneous catalyst shows robust recyclability and can be conveniently recovered and reused up to eight times without an appreciable loss of catalytic activity. Moreover, this catalyst system can realize the bond cleavage of organosolv lignin. The evidence of depolymerization was given by two-dimensional heteronuclear single quantum coherence NMR and gel permeation chromatography. Characterization of the catalyst by inductively coupled plasma, N-2 adsorption-desorption, Raman spectroscopy, scanning electron microscopy, transmission electron microscopy (TEM), high-resolution TEM, high-angle angular dark-field scanning TEM, energy-dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, and control experiments provide a fundamental understanding of the catalytic materials and the reaction pathway. Co3O4 in situ supported on a N-doped carbon matrix by the way of high-temperature pyrolysis might be a catalytically active species. Two reaction intermediates are detected and confirmed by gas chromatograph-mass spectrometry.