Molecular Dynamics Study Of Wooden Cellulose Dissolution In Hydrated Zinc Chloride And Formic Acid Cosolvent

Cellulose originated from wood and other biomass is a green and renewable natural resource.Cellulose has good intrinsic properties and abundant reserves.It can be processed into a wide range of cellulose-based products,which helps alleviate the shortage of petroleum fossil resources.At the point of the structure,cellulose is composed of the crystalline region and the amorphous region.Macromolecular chain structure of the crystalline region is arranged regularly and has strong interaction on each other,which is difficult to be dissolved and achieves phase conversion.The key to improving cellulose processing is to find the effective solvents that can break the orderly structure of tightly arranged chains in crystalline region,and this point is of great significance in promoting the further utilization of cellulose.The computational simulation gradually plays an important role in the research,and it makes up for the deficiency of experimental research at the molecular and atomic scales.Especially,molecular dynamics simulation is considered as an effective method to reflect the characteristics of the changing process of molecular systems,meanwhile,it is also applied to the dissolution of cellulose.In this paper,a deep eutectic solvent system composed of zinc chloride,formic acid and water was designed by molecular dynamics simulation,and the simulation of cellulose dissolution was carried out at 26.85~oC temperature(almost room temperature).The solvent system that could dissolve cellulose at room temperature was screened and its molecular interaction relationship was explained.Firstly,we designed three solvents by adjusting the molar ratio of the solvent components and displayed the dissolution effect of different molar ratios on cellulose through computational simulation.We mainly explored the effect of composition--structure relationship of solvent on cellulose dissolution from the aspects of the hydrogen bond number,radial distribution function and root mean square displacement.The results showed that the optimal solvent for dissolving cellulose was zinc chloride:formic acid:water in a 1:3:2 molar ratio.In the dissolution process,the hydrogen bonds of cellulose became weak while hydrogen bonds between cellulose and cellulose enhanced by forming more hydrogen bonds.Formic acid has ability to form more hydrogen bonds with the hydroxyl surface of cellulose than zinc chloride and water.From the effect of cellulose dissolution in the molecular scale,zinc chloride and water were more inclined to form metal salt hydrate that acted on cellulose together with formic acid,which is a new type of hydrated metal salt/organic acid deep eutectic solvent.Secondly,based on the results of molecular dynamics simulation theory,we further designed experiment to explore the effect of the hydrated metal salt deep eutectic solvent with the same molar ratio on cellulose dissolution at room temperature.The solvent composed of zinc chloride:formic acid:water(molar ratio of 1:3:2)could dissolve cellulose into a transparent solution at room temperature.A dot was found the disappearance of the crystal structure under the microscope,which was consistent with the simulation results.The degree of polymerization of regenerated cellulose was slightly lower than that of cellulose raw materials and the crystalline form of cellulose changed from type I to type II.The regenerated cellulose retained the integrity of its chemical structure,generated partially esterification and maintained good thermal stability(maximum thermal degradation temperature 332~oC).Finally,in order to extend the effect found in the aforementioned study,we explored the effect of cellulose structure and external field forces on dissolution by computational simulation.The results show that the degree polymerization of cellulose itself has a significant effect on the dissolution of cellulose.It could effectively promote the dissolution effect of cellulose and shorten the dissolution time by reducing the polymerization degree of cellulose.In the same solvent ratio and cellulose structure,cellulose chains was rapidly separated by increasing the microwave electric field force,leading to improve the dissolution efficiency of cellulose.The time for complete dissolution of cellulose is shortened when the field strength is5 V/nm.The above exploration provides a theoretical basis for solvents screening and combination in efficient cellulose dissolution.