Selective Depolymerization Of Lignin Into Phenolic Compounds Over Transition Metal Catalysts

Lignin is the only renewable aromatic hydrocarbon resource in nature,with the advantages of"carbon-neutral"and abundance.Catalytic depolymerization of lignin to produce aromatic compounds has become an important way for valorization,this process is increasingly concerned by researchers engaged in catalytic chemistry and biomass valorization.At present,great progress has been made on catalytic degradation of lignin,but there are still some difficulties in this field,1)relationship between lignin structure and catalytic depolymerization;2)compatibility of low-cost catalysts with catalytic activity;3)separability of monomer compounds,and son on.In view of above problems,this thesis has carried out the following researches.(1)This paper,in which six eucalyptus lignins with different structures were used as reaction substrates and Pd/C was used as catalyst,discussed the structure-activity relationship of lignin depolymerization by combination of NMR technology and catalytic methods.The linear relationship between?-O-4 linkage contents and lignin monomers yield was determined by optimizing reaction parameters of catalytic depolymerization of lignin(including reaction solvents,temperature and time).(2)This paper designed a novel Zn Mo O₄ supported on MCM-41 catalyst,which can catalyze corncob lignin to yield lignin oily products that contain 15-37.8 wt%phenolic monomers with high selectivities towards methyl coumarate(1)and methyl ferulate(2)(up to 78%).In addition,the influence of reaction parameters such as reaction solvent,temperature,time,hydrogen pressure and catalyst dosage on the process was discussed.This study achieved the separation and purification of compounds 1 and 2from lignin oily products,thus providing sustainable monomers for the preparation of functional polyether esters and polyesters.(3)This study reports that deep eutectic solvents(DESs)are used to extract C-lignin from cheap and abundant castor seed coats,which led to C-lignin biopolymer in good yields with high purity.High abundance of benzodioxane structures in various DES-extracted C-lignin samples was corroborated by HSQC NMR experiments,indicating C-lignin is highly acid-resistant.Subsequently,catalytic depolymerization of choline chloride(Ch Cl)/lactic acid(LA)DES C-lignin with Pd/C resulted in monomeric catechol derivatives in 29.6 wt%(or 81 mol%)yields with 86%selectivity toward catechylpropanol.This study provided a theoretical basis for C-lignin valorization.(4)C-lignin represents an ideal feedstock for producing catechol derivatives,this chapter reported an atomically dispersed Ru catalyst,which can serve as an efficient catalyst for the hydrogenolysis of C-lignin via the cleavage of C-O bonds in benzodioxane linkages,giving catechols in high yields with TONs up to 345.A unique selectivity to propenylcatechol(77%)is obtained,because this catalyst is capable of hydrogenolysis rather than hydrogenation.Based on the results of model compounds,the plausible pathway of C-lignin hydrogenolysis was proposed.Current demonstration validates that atomically dispersed metals can not only catalyze small molecules reactions,but also drive the transformation of abundant and renewable biopolymer.