(783c) Mechanistic Insight Into Cellulose Dissolution In Ionic Liquids

Currently, rapid growth in energy demand worldwide is deemed as promising issue in science and research community. To meet this demand, there has been considerable interest in environmentally benign energy sources. As the most abundant, biodegradable, natural material on the earth, cellulose is considered to be a viable energy source to produce biofuels. Because of the highly ordered structure and complex hydrogen-bonding network, however, cellulose is not readily soluble in water and common solvents. In the past decade, ionic liquids (ILs) have emerged as a unique class of green solvents due to relatively low melting point, negligible vapor pressure, and high thermal stability etc.  Experimental studies suggest that ILs are promising solvents for cellulose, but the mechanism of cellulose dissolution in ILs remains largely elusive.

In the present article, molecular simulations have been performed to elucidate the cellulose dissolution mechanism in ILs. A crystalline Iβ structure is adopted for cellulose and two ILs are considered, namely 1-n-butyl-3-methylimidazolium hexafluorophosphate [BMIM][PF₆] and 1-n-butyl-3-methylimidazolium acetate [BMIM][Ac]. The AMBER force field is used to mimic the cellulose and ILs, and atomic charges were estimated from quantum chemical calculations. The simulated densities, crystalline lattice constants, thermal expansion coefficients and Young’s modulus of cellulose crystal match fairly well with experimental data. The Young’s modulus along the chain direction is high due to the strong glycosidic bonds in oligosaccharide chains. The hydroxyl groups of cellulose are found to be responsible for hydrogen-bonding within cellulose, including the intra-chain O2H2∙∙∙O6 and O3H3∙∙∙O5 and the inter-chain O6H6∙∙∙O3.

Upon contact with solvents ([BMIM][PF₆], [BMIM][Ac] and water), the numbers of intra-chain and inter-chain H-bonds at the bulk layer and the intra-chain O3H3∙∙∙O5 H-bonds at the surface layer of cellulose remain nearly a constant. However, the number of hydrogen-bonds at the cellulose surface, particularly for the inter-chain O6H6∙∙∙O3 in [BMIM][Ac] deceases sharply. This is attributed to the orientational change of -OH groups at the surface by cellulose/solvent interactions. The simulation results reveal that solvation leads to the breaking of hydrogen-bonds at the cellulose surface. Among the three solvents, [BMIM][Ac] appears to have the strongest capability to break the hydrogen-bonds in cellulose. The present simulation study provides microscopic insight into the interactions of cellulose with ILs.