Preparation Of Copper-based Carbon Nitride Catalysts And Their Catalytic Performance For Oxidative Degradation Of Lignin

Lignin,the second largest biomass resource containing benzene ring structure,could be degraded into bulk chemicals and fuels to realize the high value utilization of lignin,ease the global dependence on fossil resources and achieve sustainable development.However,due to the complex structure and low reactivity of lignin,its degradation process is not clear and the yield of aromatic compounds is low,leading to the great challenge of fully utilizing lignin.Therefore,it has become a research hotspot to study the lignin degradation process and to prepare highly active catalysts to enhance the aromatic monomer yield in lignin degradation.In this paper,the oxidative degradation condition of enzymatic hydrolysis lignin under oxygen atmosphere,including alkali concentration,lignin concentration,reaction time,reaction temperature and oxygen pressure was systemically studied and the residual water phase after degradation was also analyzed thoroughly.The results showed that the increase of alkali concentration was beneficial to the dissolution of lignin and the polarization of ether bond,thus promoting the conversion of lignin.High lignin concentration affected the mass transfer and the degradation,while low lignin concentration reduced the oxygen content in the system.Higher reaction temperature and shorter reaction time were beneficial to the accumulation of aromatic monomer.Aldehydes and ketones in the products are relatively stable and not easy to be oxidized to phenolic acids.G-type and H-type products have better stability than S-type products,and all the aromatic products might be further oxidized into small molecule carboxylic acids.Catalyst with cuprous oxide supported on carbon nitride?Cu₂O/C₃N₄?was prepared by a one-step calcining method.The structure of the catalyst was analyzed by means of XRD,XPS,SEM,TG,FT-IR,and the effects of cuprous oxide load,catalyst dosage and thermal oxidation modification on the catalytic performance for lignin oxidative degradation were investigated.When the cuprous oxide load was 1.2 wt%and the catalyst dosage was 3 wt%of lignin mass,the yield of monophenols could be increased from 5.8%to 12.1%.In order to improve the oxidation capacity of catalyst,enhance the stability of catalyst,and further improve the yield of lignin degradation,copper oxide catalyst coated with carbon nitride?Cu O@C₃N₄?was prepared.Nitrogen doping of copper oxide could effectively promote the coating effect of carbon nitride on copper oxide,and further increased the yield of monophenols to 16%.GC-MS,FT-IR and GPC were used to analyze the degradation products and explore the degradation process of lignin.Lignin is firstly degraded under alkali and oxygen to obtain intermediate products,and then the intermediate products were converted into free radical products under the synergistic action of copper oxide and oxygen.The free radical products are soon converted into aromatic monomer compounds,which can be further oxidized into smaller carboxylic acids.