Dual Regulatory Mechanism Of Ionic Liquids Dissolving Water Molecules In Ligni

Lignocellulose,as the most abundant biomass resource on earth,has broad application prospects.One of the main components of lignocellulose is lignin.As the only biomass resource containing benzene ring structure in nature,it has become the raw material of various platform compounds,which has attracted extensive attention.However,lignin often coexists with cellulose through complex cross-linking structure and has strong chemical and biological resistance to degradation.Pretreatment is usually required in the early stage of high-value utilization.The traditional lignin separation or processing technology requires harsh conditions.Therefore,it is necessary to find and use renewable solvents to dissolve lignin,so as to realize the recycling of lignin.In recent years,ionic liquids(ILs),as a new solvent,have been proved to have the advantages of strong designability and stable thermodynamic properties,and show obvious advantages in biomass separation.However,there are still some bottleneck problems in the process of industrial application,including high viscosity,high cost and low selective separation efficiency.The experimental research shows that adding an appropriate amount of water to ILs not only effectively reduces its viscosity and cost,but also shows good solubility to lignin.Therefore,ILs aqueous solution can be used as an effective solvent for lignin dissolution.Based on this,this paper studies the dual regulation of water on lignin dissolution in ILs at the molecular level through molecular dynamics simulation,and makes an in-depth study on the effect of water content on the interaction between anion and anion and lignin,as well as the dissolution mechanism of different lignin models in different types of ILs([Emim][Cl],[Emim][OAc],[Bmim][OAc]and[Bmim][Cl]).This study provides ideas for the design of ionic liquid water system for biomass economic pretreatment.The specific results of this paper are as follows:(1)The micro mechanism of ionic liquid dissolving lignin was clarified.In the process of lignin dissolution,anions are distributed around the hydroxyl groups of lignin and located in the first solvation layer,and the intermolecular interaction is mainly electrostatic;The cation is distributed in the second solvation layer outside the anion and acts on the position of lignin aromatic ring,mainly van der Waals.Anion and cation act on different positions of lignin respectively,which plays a synergistic role in the dissolution of lignin.(2)The double effect of water on the dissolution of lignin in ionic liquid was revealed.In the process of dissolving lignin by ionic liquid,the addition of a small amount of water can effectively destroy the ion association in pure ILs,that is,produce more free cation and anion.These free ions can be more easily distributed around lignin,so as to further promote the dissolution of lignin.However,adding too much water will cause excessive solvation of anions and cations,and make water gradually change from cosolvent to antisolvent.(3)The effects of different types of ionic liquids on lignin were summarized.The growth of imidazole cycloalkyl side chain of cations increases the spatial resistance of the interaction between cations and lignin,and the interaction between cations and lignin can be significantly reduced.Therefore,the long cationic alkyl chain is not conducive to the dissolution of lignin.There are a large number of relatively stable hydrogen bonds between ILs and lignin.Because there are two oxygen atoms on[OAc]~-,it can form more hydrogen bonds with lignin.