Investigation On Cleavage Of Lignin-related β-O-4Linkage

Lignin depolymerization is one of the most significant barriers to reaching the fullpotential of lignocellulosic biofuels as fossil fuel replacements. The problem stems fromthe fact that lignin comprises15-25%of the mass found in lignocellulose and up to40%of the energy content. For lignocellulosic biofuels to be sustainable, chemistry must bedeveloped to convert lignin into small molecules that can be upgraded into a fuel stream.In this thesis, the selective reductive cleavage of a lignin-related β-O-4linkage wascompleted with Fe salts, ionic liquids or Ni-Fe as the catalysts or auxiliary in the case ofusing LiAlH₄. The main contents are as follows:The selective reductive cleavage of a lignin-related β-O-4linkage was completedwith Fe（acac）₃（ferric acetylacetonate） as the catalyst precursor, and LiAlH₄as thereducing agent. The reaction occurred at the C-O bonds without reductive cleavage ofthe π-bonds in phenyl rings. After the reaction conditions including temperature,auxiliary, solvent and catalyst were optimized, the obtained optimal conditions is that:140oC as the reaction temperature and NaOtBu as the base. Under the optimalconditions, the cleavage of a series of lignin-related β-O-4linkage model compounds（2-aryloxy-1-arylethanols） proceeded smoothly, and gave the phenol products in highyields.The selective reductive cleavage of benzyl phenyl ether and a lignin-related β-O-4linkage was completed in ionic liquids. The obtained optimal conditions is that:0.5mmol benzyl phenyl ether,1mL [BMIM]Cl,2mmol AlCl₃,120oC，20h. Under theoptimal conditions, The reactions of various benzyl phenyl ethers afforded benzylchloride and phenol products in high yields, and the two products were obtained in up to76%and84%respectively, which seems to present a method for the deprotection ofphenolic hydroxyl group. However, the cleavage of2-aryloxy-1-arylethanols containinga lignin-related β-O-4linkage gave the desired products in low yields, and the obtainedyield of phenol is up to28%.The selective reductive cleavage of a lignin-related β-O-4linkage was completed with Ni-Fe as the catalyst under hydrogen gas, which provided the basis of the theoryfor the depolymerization of lignin. In previous literatures, under hydrogen gas, thereductive cleavage of the π-bonds in phenyl rings occurred as the main reaction, whileno hydrogenation of the phenyl rings was observed. After the reaction conditionsincluding temperature, auxiliary, solvent and catalyst were optimized, the obtainedoptimal conditions is that: SIPr·HCl as the ligand, sym-trimethylbenzene as the solvent,140oC as the reaction temperature and NaOtBu as the base. Under the optimalconditions, the cleavage of a series of lignin-related β-O-4linkage model compounds（2-aryloxy-1-arylethanols） underwent this transformation smoothly, and gave the phenoland acetophenone products in high yields.