Understanding Lignin Treatment In Dialkylimidazolium-based Ionic Liquid–water Mixtures

In the utilization of biomass resources, how to remove lignin is an important part of the extraction of cellulose. Now the most widely used method is acid and alkali method, but this method of environmental pollution, but also seriously changed the structure of lignin. In recent years, as a new green solvent, ionic liquid has the advantages of low melting point, low vapor pressure, and high solubility in biomass. It has been applied to the separation of lignin and biomass. At the same time because of its high viscosity, high cost and limited its development in this field. So it was proposed to add the right amount of water to reduce the viscosity of the solvent, while saving costs.But the effect of the addition of water on the removal of lignin is not clear. Due to the ionic liquid is breaks the natural macromolecular lignin into lignin fragments after dissolution, so in order to further study of this system, our research group studies of ion liquid water system of wood element dissolution process and ionic liquid- water system of wood pigment degradation process, and then explore the water join effect on the removal of lignin. In the study of lignin dissolution process, we proceed from the point of view of thermodynamics, so that we can theoretically explore the dissolution process of lignin. In the study of lignin dissolution process, we proceed from the point of view of thermodynamics, so that we can theoretically explore the dissolution process of lignin.Through the analysis of the experiment, the following conclusions are obtained:(1)The process of lignin dissolution in ionic liquid was exothermal, indicating that increasing temperature didn’t favor lignin dissolution.(2)The addition of water dramatically influences the enthalpy of lignin dissolution, thus confirming that water participates in the interactions between the ionic liquid and lignin.(3)The addition of water also accelerates the thermodynamic stable state of the mixed system.(4)pH, IL type, and IL content were found to greatly influence the degradation of lignin and the structure of regenerated lignin. 1-Butyl-3-methylimidazolium methylsulfonate–water mixtures with low pH facilitated lignin depolymerization but destroyed the regenerated lignin substructure. Regenerated lignin with low molecular weight and narrow polydispersity index was obtained using a 1-butyl-3-methylimidazolium acetate-based system.(5)Water addition inhibited lignin depolymerization at 50–100 wt% IL content, except for 70 wt% 1-butyl-3-methylimidazolium chloride–water mixture. Compared with pure IL treatment, obvious differences were observed in the structure of regenerated lignin with IL–water treatment.