Theoretical Studies On Dissolution And Depolymerization Mechanisms Of Lignin In Ionic Liquids

Lignin consisted of sinapyl,coniferyl,and p-coumaryl alcohol units,is cross-linked phenolic polymer containing large numbers of methoxylated phenylpropanoid subnuits,which links together through relatively stable C-0 and C-C linkages,including ?-O-4,4-0-5,5-5,?-5,?-1 type linkages.The ?-O-4 linkage is the dominate one,accounting for 48-60%of the total linkages in lignin.Currently,a few methods of lignin dissolution and catalytic depolymerization based on the ?-O-4 linkage lignin model compound have been reported,which almost involved high-cost,acids,caustic alkali,volatile toxic solvents and the condensation of carbon-carbon bonds that can bring about negative effects on the environment and decrease the yield of depolymerization.These seriously hinder the development of lignin chemistry in industry.Ionic liquids(ILs)have attracted considerable attention as new green solvents,primarily in the process of lignocellulosic biomass.In 2002,Rogers and coworkers make a ground breaking discover that cellulose can dissolve in imidazolium ionic liquids.Under the joint efforts of many research institutions at home and abroad,the studies of lignin dissolution and degradation in ionic liquids have made important progress,and brought about large-scale applications.In recent years,ionic liquids have been used to dissolve and degrade lignin.More recently,the ionic liquid of 1-allyl-3-methylimidazolium chloride([Amim]Cl)has been found to be the most efficient relatively for dissolving lignin;the acidic IL[Amim]Cl can be used to catalytically cleavage the(3-0-4 bond in the lignin model compounds and true lignin under mild conditions,which can significantly improve the efficiency of conversion of lignin.Furthermore,the research of lignin conversion shows that ionic liquids act as both reaction medium and catalyst,and the yield of its depolymerization can reach about 70%,which is significantly different from lignin conversion in the traditional acid and alkali mediums.Thus,ionic liquids have attractive application prospect in the field of biomass conversion.The usage of ionic liquids is expected to obtain high value-added chemicals and biofuels by changing the technological process of the traditional lignin conversion.In the dissertation,we chose veratrylglycerol-?-guaiacyl ether(VG)featuring the?-O-4 linkage as a lignin model compound to investigate the dissolution of lignin in[Amim]Cl ionic liquid by molecular dynamics(MD)simulation and density function theory(DFT)and the cleavage mechanism of its ?-O-4 bond over acidic ionic liquid[HMIM]Cl catalysts by DFT.1.Theoretical study on the interaction mechanism of the model compound of ?-O-4 type lignin in 1-allyl-3-methylimidazolium chloride ionic liquid.Some previous studies have shown that imidazole chloride[HMIM]Cl and 1-butyl-3-methylimidazolium hydrogen sulfate([BMIM]HSO4)ionic liquids are the most effective ionic liquids for lignin dissolution.We have performed MD simulations combined QM calculations to understand the mechanism of lignin dissolution using VG in[Amim]Cl IL at the molecular level.From the calculation results,it is indicated that VG interacts with[Amim]Cl mainly by hydrogen bonds and ?-? stacking,and the interaction of[Amim]Cl-VG is stronger than that of VG-VG,indicating[Amim]Cl can dissolve lignin.In addition,IL anions disrupt lignin model intramolecular H-bonds,and the interaction of VG with imidazolium cation is weaker than that of VG with chloride anion,which demonstrates that the IL anion plays a major role in lignin dissolution.2.Cleavage of the ?-O-4 bond of lignin model mompound catalyzed by an acidic ionic liquid,1-H-3-methylimidazolium chloride:a DFT mechanistic study.We chose veratrylglycerol-?-guaiacyl ether(VG)featuring the ?-O-4 linkage as a lignin model compound to investigate the cleavage mechanism of its ?-O-4 bond over acidic ionic liquid(IL)1-H-3-Methylimidazolium Chloride([HMIM]Cl)catalysts by density function theory(DFT).The depolymerization pathway involves dehydration and hydrolysis process.The dehydration process utilizes the acidic proton of the imidazolium cation to attack the a-hydroxyl group to provide help for the leave of H2O molecule.The dehydration revealed that protonation with the most acidic N1-H proton is more favorable than the rest of protons(C2-H,C4-H and C5-H proton).This is ascribed to the acidity of different protons of IL.In the study,the roles of the cation and anion of[HMIM]Cl are unraveled.The characterized roles explain why[HMIM]Cl medium can accomplish the lignin depolymerization.In addition,we propose and confirm a similar mechanism for preventing VG condensation by adding formaldehyde in its depolymerization.These results provide help for understanding the depolymerization of?-O-4 type lignin model compound catalyzed by[HMIM]Cl and predict the feasibility of the depolymerization for adding formaldehyde to improve the yield of depolymerization.