Interactions Between Lignin And Cellulose Via Molecular Dynamics Simulations

Lignocellulosic biomass is one of the most abundant renewable resources on the earth and its utilization is the important direction of future energy development.Lignocellulosic is formed by tightly wrapping highly-crystalline cellulosic fibers with hemicellulose and lignin.The tight and complex structures result in natural biomass recalcitrance,thereby increasing difficulty in high-efficient utilization of biomass.Therefore,pre-treatments are commonly used in industrial conversion for biomass component separation to improve the preparation efficiency for high-value chemicals and reduce their costs.Since heating is usually the vital step in various biomass pre-treatments and thermal conversion processes,and lignin is generally not sufficiently degraded by such pre-treatment and residual lignin will deposit and combine with cellulose,influencing subsequent high-value utilization.Therefore,studying the characteristics of cellulose-lignin and lignin-lignin interactions at different temperatures is of significance for revealing the fundamental causes for biomass recalcitrance.In this paper,a long-term molecular dynamics(MD)simulations were carried out for the cellulose-lignin complex,lignin alone system and cellulose alone system at three different temperatures.Relationships between factors,including the distribution of lignin on the cellulose surfaces,lignin self-aggregations,and solvation properties of cellulose surfaces,were systematically investigated along with the temperature in the dynamics process.Based on the simulation results,influences of temperature on the intensity of the cellulose-lignin interactions were summarized.These are intended to reveal the essence of biomass component separation and further optimize the pre-treatment and utilization processes in the future.The simulations demonstrate that the lignin-cellulose interactions at different temperatures are mainly influenced by two factors:the degree of solvation of cellulose surfaces and that of lignin self-aggregation.At a normal temperature,cellulose and lignin always maintain balanced interactions in the dynamic processes.At 400K,as the solvation effect of the cellulose surfaces reduces due to the increased temperature,lignin can be rapidly deposited on the hydrophobic cellulose surfaces,contributing to the enhanced VDW force compared to those at a normal temperature.However,with the constant enhancement of lignin self-aggregation at that temperature,the distribution of lignin on the cellulose surfaces declines continuously,that is,the interaction weakens,whereas,lignin can still be attached to the cellulose surfaces.At 550K,the high temperature exposes more hydrophobic cellulose surfaces and impairs lignin self-aggregation.Lignin cannot be stably combined with cellulose,so the distribution of lignin on the cellulose surfaces decreases significantly and the interaction weakens.If it is simulated for a much longer time,the two might eventually be separated at the molecular level.