Study On The Selective Oxidation Of Lignin To Prepare Aromatic Chemicals

The application prospect of using biomass resources as raw materials to produce liquid fuels or chemicals is becoming increasingly clear,which provides an effective path for the green transformation and development of the refining and chemical industry.Biomass contains three main components:cellulose,hemicellulose and lignin.Cellulose and hemicellulose have been applied in the industry to some extent due to their simple and clear structures.Lignin is rich in aromatic structure,but due to its heterogeneity and complex three-dimensional network structure,most of lignin is still burned as fuel to recover energy,resulting in waste of aromatic ring resources.Based on this,the main goal of this thesis is to produce aromatic compounds from lignin with wet oxidation,and improve wet oxidation by reducing the molecular weight of lignin and catalyst catalysis.The main research details and conclusions of this thesis are as follows.(1)Employing choline chloride as a hydrogen bond acceptor,three various deep eutectic solvents(DES)were prepared for pretreatment of lignin.The effects of different factors on the structure and molecular weight of lignin were inspected,and then the regenerated lignin was wetly oxidized.The oxidative conversion rate and aromatic product yield before and after pretreatment were compared.The results showed that with the extension of the pretreatment time,the molecular weight of lignin was greatly reduced after pretreatment at different temperatures,and the wet oxidative conversion rate was as high as 86.1 wt%.At high temperatures,the lignin obtained after acidic/alkaline DES pretreatment was wetly oxidized to produce a lower yield of aromatic products,and a relatively high yield was obtained in alcohol-based DES.The break of ether bonds of lignin during the pretreatment process,and then a part of aromatic products were obtained in advance,which was also considered to be the main reason for the reduction of the molecular weight of lignin.(2)Metal oxides and catalyst supports were screened,and then a series of supported metal oxides were prepared for catalytic wet oxidation of enzymatic lignin.The results showed that when nano-hydroxyapatite was loaded with 10wt% Mn,the optimal yield of aromatic products was 26.0 wt%.However,after the second metal was loaded,the yield of aromatic ring products was significantly reduced due to the increase of surface oxygen concentration and the corresponding decrease in pore sizes of the catalyst.When Mn was loaded as the second metal,the excessive oxidation of aromatic ring products was caused by Sr during oxidation,which indicated that manganese oxide has an inhibitory effect on metal oxides with strong catalytic effect.(3)Combined with the information obtained by characterization and products,and referring to the literature,it is believed that the preparation of guaiac-based products during the wet oxidation of lignin is mainly due to the cleavage of the base-catalyzed β-O-4 bond and the subsequent breaking of the C-O bond caused by the participation of oxygen to form small molecule chemicals.