Study On C-C Bond Cleavage Of Lignin And Models Via Baeyer-Villiger Oxidation And Beckmann Rearrangement And Photoinduced By Ir-complex And CeCl₃

Oil is a non-renewable resource,and the search for sustainable alternatives to petroleum is a must for the sustainable development of society.Lignocellulose is the most abundant carbon resource biomass on earth.It is mainly composed of cellulose,hemicellulose and lignin.Among them,lignin is the only non-fossil-based resource containing aryl groups in nature,and the second most renewable biomass resource,next to cellulose.However,lignin has not been widely used,and most of it is discharged as papermaking wastewater,or incinerated,which not only causes serious environmental pollution,but also wastes a great resource of nature to human beings.Lignin is a biological macromolecule mainly composed of three phenylpropane structural monomers linked by C-O bond or C-C bond.It crosslinks with cellulose and hemicellulose through covalent bond and hydrogen bond to form a three-dimensional cross-linked structure,so that the plant cell wall has sufficient strength to protect plant cells.The complex structure of lignin,difficult extraction and separation properties,and stable chemical bonds make its degradation application very difficult.At present,due to the low energy barrier of C-O bonds,the degradation of lignin is mainly concentrated on the cleavage of C-O bonds,and various effective C-O bond cleavage methods have been developed,such as hydrogenation,oxidation,disproportionation,acid hydrolysis,and alkali hydrolysis.The inertness of the C-C bond and the higher energy barrier have great challenges in cleavage.At present,the cleavage of C-C bonds is mainly concentrated on heat-induced systems?metal catalysis such as vanadium,copper,iron,ruthenium,osmium,iridium?and light-induced systems?metal catalysis such as vanadium and iridium and some composite nanomaterials?.The development of highly active,selective,and mildly C-C bond activation systems remains challenging for lignin conversion.This work mainly focuses on the following two aspects:?1?Application of organic name reaction in lignin C-C bond cleavageOrganic chemistry,especially organic name reactions,is an important branch of chemistry.These reactions involve the forming and breaking of various types of chemical bonds?including C-O bond and C-C bond?.Classical organic chemical reactions are characterized by mild conditions and easy control.In chapter II,we use the classical organic name reaction,Baeyer-Villiger oxidation reaction to,convert C-C bond into ester or acetal,and then alcoholysis to achieve indirect cleavage of C-C bond of oxidized lignin models.In chapter III,we use the Beckmann rearrangement reaction to convert the inert C-C bond into amide,which is then hydrolyzed to efficiently break the bond under mild conditions.Nitrogen substituted aromatic compounds,for example,aniline,benzonitrile,benzamide,C-2 substituted oxazole,were obtained.?2?Photoinduced alkoxygen radical generation,highly selective lignin C-C bond cleavagePhotocatalysis has been applied to small molecule activation reactions due to its mild reaction conditions and efficient conversion,and it has also been involved in the cleavage of C-O bond and C-C bond of lignin and models.However,photocatalytic lignin C_?-C_?bonds selective and efficient cleavage and two products obtained is yet to be developed.In 2016,R.R.Knowles calculated theoretically that alkoxy radicals can weaken the bond energy of?-C-C bond.The formation of light-induced alkoxy radicals has become a new idea for activating C-C bonds.In chapter IV,we use the iridium complex to undergo proton-coupled electron transfer?PCET?,photoinduced lignin?-O-4 and?-1 bond?-OH to form alkoxyl radicals,further inducing C_?-C_?selective cleavage.In chapter V,we use CeCl₃ to undergo ligand-to-metal charge transfer?LMCT?,photoinduced lignin?-O-4 and?-1 bond?-OH to form alkoxyl radicals,further inducing C_?-C_?selective cleavage and amination.The two photocatalytic C-C bond cleavage method has the advantages of high selectivity of the bond position,high product selectivity and mild conditions.