Molecular Simulation Study On Dissolution Of Lignocellulosic Biomass In Ionic Liquids

Lignocellulosic biomass is one of the most abundant renewable resources on the earth and its utilization is the important direction of future energy development.It is mainly composed of cellulose,lignin and hemicellulose,in which cellulose has the highest proportion of the content.Due to the structural robustness,pretreatment process such as dissolution is necessary before its further conversion and utilization.With plenty of appealing properties as solubility and stability,ionic liquid(IL)is regarded as the "star solvent" for cellulose these years.However,the reason why ILs can dissolve cellulose is under debate,and there is still no systematic explanation.In this work,realistic cellulose models were constructed and complete dissolution process was obtained through large-scale molecular dynamics simulation,through which the mechanism of dissolution of cellulose in different kinds of ILs was revealed.The effects of cation saturation on cellulose dissolution and the interaction between IL and lignin were also explored.It is hoped that this study can provide a theoretical basis for understanding the microscopic dissolution process and developing new IL solvents.The main contents and results are as follows:(1)Simulation of ILs dissolving cellulose bunch.In this part,a cellulose bunch containing 7 single glucose chains(degree of polymerization = 8)was constructed,and molecular dynamics simulations were carried out in[Emim][Cl],[Emim][OAc],[Bmim][Cl]and H2O for 500ns.It was found that the dissolution rate of cellulose in IL was in this order,[Emim][OAc]>[Emim][Cl]>[Bmim][Cl],which was in agreement with the experiment.[OAc]-can form three different kinds of H-bonds within cellulose chains which can provide enough gaps for separation.[Cl]-cannot effectively divide the cellulose chains and this is why[OAc]-is more effective.In addition,a synergistic mechanism of cation and anion dissolving cellulose was proposed.The anions initially form H-bonds with hydroxyl groups with an insertion into the cellulose strands and cations stack to the side face of the glucose rings.As more and more anions bind to the cellulose chains,cations start to intercalate into cellulose bunch due to their strong electrostatic interaction with anions and Van-der-waals interaction with the cellulose bunch.Then cellulose dissolution begins.(2)Simulation of IL dissolving cellulose microfibril.A cellulose microfibril containing 36 single chains(degree of polymerization = 40)was set up and simulated in[Emim][OAc]for 3?s.Through analysis of structure changes of cellulose microfibril during dissolution,an anticlockwise twist was observed,and the twist would occur in a very short period of time,earlier than dissolution.The dissolution route was also investigated,and cellulose micro fibril was dissolved in IL through a gradual peeling-off process of single chains.The chain at the junction of water and hydrophobic surface was firstly peeled off,starting from the reducing end.The energy and position information of ILs and cellulose was analyzed,and then the interaction model of IL and cellulose was put forward.The[OAc]-anion interacts with cellulose hydroxyl groups in the horizontal direction,and the[Emim]+ cation preferred to make contact with the hydrophobic surface of cellulose microfibril.(3)The controlling mechanism of the unsaturated structure of cations on the dissolution of cellulose in ILs.The changing process of the cellulose bunch in[Bmim][OAc],[Bpyr][OAc],[Bpy][OAc]and[Bpip][OAc]were simulated.It was found that cellulose can only be dissolved in ILs containing unsaturated cations,in accordance with experimental results.The interaction between different cations and cellulose was studied.The influence of heterocyclic ring was analyzed by quantum chemistry calculation and the effect on the mass transfer was also investigated.It is found that the mechanisms of unsaturated heterocyclic structure include two aspects.One is the structure factor:the ? electron delocalization of unsaturated heterocyclic ring makes the cation more active to interact with cellulose and provides more space for acetate anions to form hydrogen bonds(H-bonds)with cellulose.The other is the dynamic effect:the larger volume of cations with saturated heterocyclic ring result in a slow transfer of both cations and anions,which is not beneficial to the dissolution of cellulose.(4)The interaction and interface structure of ILs and lignin.A softwood lignin model based on experimental data was established,and simulation of the lignin model with[Emim][Cl],[Bmim][Cl],[Emim][OAc],[Choline][OAc]and[Choline][Gly]was carried out.It is found that ILs can form a relatively stable distribution around lignin.The distribution of anion was in the first solvation shell,and the main interaction energy comes from electrostatic interaction;the cation distributed in the second solvation shell,interacting with lignin mainly through Van-der-waals interaction.The cation and anion interact with lignin at different positions and dissolve lignin together.IL containing[OAc]-can form more H-bonds with lignin and IL containing[Gly]-can form a wide range of conformations with large H-bond number,which explains why[Emim][OAc],[Choline][OAc]and[Choline][Gly]dissolve lignin better than[Cl]-based ILs.