Study On The Depolymerization Of Lignin Model Compounds By Cobalt-based Nitrogen Doped Carbon Materials With Catalytic Oxidation

Heterogeneous catalysis is one of the effective and green methods to realize the efficient andhighly selective oxidation conversion of lignin to high value-added chemicals under mild conditions.Traditional inorganic oxides and their supported metal catalysts suffer several drawbacks in terms of experimental points such as low catalytic activity,low lignin degradation efficiency and required harsh conditions.Therefore,the development of new catalysts is highly desirable.Compared with the traditional inorganic oxide-supported metal catalyst,the noble-metal-free-based nitrogen-doped mesoporous carbon materials is easier to adjust its catalytic activity and stability.Based on this,we applied noble-metal-free-based nitrogen-doped mesoporous carbon materials catalysts in the oxidative cleavage of lignin and lignin model compounds to prepare high value-added aromatic compounds using molecular oxygen as the oxidant.Firstly,VB₁₂@C was prepared via impregnation method with natural vitamin B₁₂ containingcobalt,nitrogen and carbon as the precursor and activated carbon as the support.And then pyrolysis of VB₁₂@C at different temperatures,the as-synthesized materials are labeled as VB₁₂@C-X.（X represents the pyrolysis temperature）.The catalyst can achieve the oxidative C-C and C-O cleavage of Ligninβ-O-4model compounds to phenols and aromatic esters using molecular oxygen as the oxidant under mild reaction conditions.The catalyst can achieve the oxidative C-C and C-O cleavage ofβ-O-4 ketones even at room temperature using 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.Characterization of the catalyst by inductively coupled plasma,N₂ 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.Subsequently,we prepared heterogeneous single-atom cobalt catalysts supported onnitrogen-doped mesoporous carbon materials by hard template method with natural vitamin B₁₂ as precursor.The catalysts exhibits excellent catalytic activity toward the oxidative cleavage of a variety of1,2-diols into esters,ketones or aldehydes using molecular oxygen under mild conditions.The robust catalyst can be reused at least seven times without regeneration treatment.The formation of highly dispersed active Co-Nx sites was demonstrated by catalyst characterization and KSCN poisoning experiment.Through a large number of control experiments,kinetics and isotope-labeling experiments,the oxidative cleavage reaction processes of monosubstituted and internal diols were studied respectively:mechanistic insights into monosubstituted diols indicate a sequence reaction including stepwise oxidation/nucleophilic addition/C-C bond cleavage;internal diols followed type II mechanism involves the formation of a monodentate complex between diol and active oxidant species,which then oxidatively collapses to the carbonyl product in a two-electron oxidation.