Catalytic Oxidation Of Lignin To Aromatic Compounds By Oxygen

Under the"double-carbon"goal,the increasing depletion and pollution of fossil energy have become the main problems hindering the current economic and social development.It is urgent to find a clean and renewable energy to achieve green transition,and biomass is one of expected the energy resources.Biomass is mainly composed of cellulose,hemicellulose,and lignin.Among them,lignin contains rich aromatic structures and has important research significance.However,due to the complexity of its structure and the problem of solvent pollution in the transformation process,the conversion of lignin does not achieve high-value utilization.Consequently,in response to this problem,this paper uses environmentally friendly water and ionic liquids as the main solvents to catalyze the oxidation of lignin to achieve green and efficient preparation of aromatic chemicals from lignin.The main research contents and conclusions are as follows:（1）Oxygen was used as oxidant,imidazolium-based ionic liquid aqueous solution was used as reaction solvent,and Cu Cl₂was used as catalyst to catalyze the oxidation of lignin.The weak bonds in the structure of lignin were broken,and aromatic rings were retained to prepare aromatic phenolic compounds.The effects of different anions in the ionic liquids,different carbon chains on the imidazole ring,oxygen pressure,reaction temperature,p H and other factors on the catalytic oxidation of lignin were explored.The results showed that 10.1 wt%aromatic aldehydes and 45.4 wt%of small-molecule fatty acids or aldehydes were obtained using[Bmim]Cl-H₂O as solvent at conditions of 150℃,1.5 MPa O₂,p H=4 and a reaction time of 2 h.In addition,the ionic liquid and Cu Cl₂had synergistic catalytic effect on lignin conversion,and catalyst system could be reused.（2）Using oxygen as oxidant and water as solvent,the catalytic oxidation of lignin was carried out under the action of sodium hydroxide.The effects of sodium hydroxide concentration,reaction temperature,reaction pressure,and different catalysts on the preparation of aromatic phenolic and benzene carboxylic acid by alkaline oxygen oxidation of lignin were investigated.The results showed that 21.0 wt%of aromatic phenolic compounds（mainly vanillin,syringaldehyde,and acetosyringone）and 3.6 wt%of benzene carboxylic acid（mainly benzene pentacarboxylic acid and mellitic acid）were obtained using Mn O₂as catalyst at conditions of 2.0 mol/L Na OH,160℃,1.0MPa O₂,and a reaction time of 1 h.The catalyst Mn O₂could be reused,and the catalytic performance had no obvious change after four cycles.Combined with the work of this research group,the oxidation of lignin to benzene carboxylic acids under alkaline conditions was explored.Under optimal conditions（reaction temperature,260°C;O₂pressure,5 MPa;Na OH concentration,2.0 mol/L;reaction time,30 min）,8.1 wt%yield of benzene carboxylic acids was obtained.（3）The original lignin,the reaction residue and the liquid product after the reaction were characterized by FT-IR,¹³C-NMR,HSQC-NMR,etc.It was found that the two catalytic oxidation systems in this paper could both cleavage weak bonds such as ether bonds to prepare aromatic compounds.In addition,the existence of polycyclic structures in the lignin structure was verified,and the reason for the formation of benzene carboxylic acids during the oxidation process was explained.At the same time,combined with the product information,the path for the formation of aromatic phenolic products during the alkaline oxygen oxidation of lignin was proposed:theβ-O-4 type structure underwent deprotonation,dehydration,oxidation,and reverse aldol condensation to achieve the cleavage of C-O and C_α-C_β,thereby obtaining aromatic aldehydes.