Degradation And Increment Of Lignin And Models Under Mild Conditions

The rapid consumption of energy brought by population growth and economic development has caused a crisis of non-renewable fossil energy such as coal and oil.Replacing fossil resources with renewable resources to meet the energy needs of human production and life has become a strategic goal of sustainable development.Biomass is the fourth largest energy source on earth after coal,oil,and natural gas,and it is the most abundant renewable resource on the earth.Lignocellulose is the most carbon-containing biomass on the planet consisting of cellulose,hemicellulose and lignin,which has long been considered the most promising alternative to fossil resources.Among them,lignin is a natural macromolecule polymer with a three-dimensional network structure,and is the only aromatic-containing renewable energy source in the earth.Lignin is the second most renewable biomass in nature only after cellulose and was composed of three phenylpropanyl aromatic units linkage by C-C and C-O bonds.Due to the substituent and substitution positions on the three structural units are different,and each different phenylpropane unit connected at different positions,the structure of natural lignin is complex and cannot be expressed by a definite chemical formula,thence the utilize of lignin is far behind cellulose,and most researches are studied on lignin model compounds.As the research on lignin degradation is gradually deepening,scientists start to consider how to degrade lignin under milder conditions and obtain functional high-value platform chemicals.Based on our group’s previous work,this paper is mainly carried out from the following two aspects.（1）Effective C-O bond cleavage by cooperation of Lewis acid and nucleophile:Heteroatom-substituted aromatic monomer compounds are of great significance in pharmacy,biology,and organic synthesis.It is a valuable research work to obtain functional aromatic monomer products from the degradation of lignin.After successfully obtained N-functionalized and Br-functionalized products from lignin model compounds in our group’s previous work,in the second chapter we attempted to introduce sulfur atom into lignin degradation products under the condition that AlCl₃ as Lewis acid and thiol as nucleophile,the product dithioacetal was obtained.Dithioacetals are usually used as precursors to synthesize pharmaceuticals or bioactive species,meanwhile,it is also admitted act as diversity intermediates in organic synthesis.Dioxane-extracted pine lignin has a significantly smaller molecular weight after react under this system,which proves that it can degrade lignin.（2）Selective cleavage of C-C bonds of lignin under photocatalysis:Due to the mild reaction conditions and high reaction efficiency,photocatalysis has become a promising strategy in organic chemistry and it has also been used in the degradation of lignin in recent years.In our group’s previous work,we have successfully used iridium and cerium as photocatalysts to break the C-C bond in lignin.In the third chapter,we tried to use cheaper and more environmentally friendly compounds as catalysts to break the C-C bond of lignin.Related work is ongoing and preliminary results have been obtained.