J. Phys. Chem. B 2010, 114, 4293–4301
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Understanding the Interactions of Cellulose with Ionic Liquids: A Molecular Dynamics Study Hanbin Liu,†,‡ Kenneth L. Sale,†,§ Bradley M. Holmes,†,‡ Blake A. Simmons,†,‡ and Seema Singh*,†,‡ Deconstruction DiVision, Joint BioEnergy Institute, EmeryVille, California 94608, Biomass Science and ConVersion Technology Department, Sandia National Laboratories, LiVermore, California 94551, and Biosystems Research Department, Sandia National Laboratories, LiVermore, California 94551 ReceiVed: December 11, 2009; ReVised Manuscript ReceiVed: February 8, 2010
Ionic liquids (ILs) have recently been demonstrated to be highly effective solvents for the dissolution of cellulose and lignocellulosic biomass. To date, there is no definitive rationale for selecting ionic liquids that are capable of dissolving these biopolymers. In this work, an all-atom force field for the IL 1-ethyl-3methylimidazolium acetate [C2mim][OAc] was developed and the behavior of cellulose in this IL was examined using molecular dynamics simulations of a series of (1-4) linked β-D-glucose oligomers with a degree of polymerization n ) 5, 6, 10, and 20. Molecular dynamics simulations were also carried out on cellulose oligomers in two common solvents, water and methanol, which are known to precipitate cellulose from IL solutions, to determine the extent and energetics of the interactions between these solvents and the cellulosic oligomers. Thermodynamic properties, such as density and solubility, as well as the two-body solute-solvent interaction energy terms, were calculated. The structural and dynamic behavior of solutions was analyzed and the conformations of cellulose oligomers were compared in ionic liquid and water mixtures. It was found that the interaction energy between the polysaccharide chain and the IL was stronger than that for either water or methanol. In addition to the anion acetate forming strong hydrogen bonds with hydroxyl groups of the cellulose, some of the cations were found to be in close contact with the polysaccharides through hydrophobic interactions. These results support the concept that the cation may play a significant role in the dissolution of cellulose by [C2mim][OAc]. It is also observed that the preferred β-(1,4)-glycosidic linkage conformation of the cellulose was altered when dissolved in [C2mim][OAc] as compared to that found in crystalline cellulose dispersed in water. To our knowledge, this report is the first theoretical study that addresses the key factors in cellulose dissolution using an ionic liquid. Introduction Ionic liquids (ILs), or room temperature ionic liquids (RTILs), are organic salts that are typically fluid below 100 °C. They are considered to be “green solvents” as they have negligible vapor pressure. ILs have been suggested as potential replacements for volatile organic compounds (VOCs) in industrial separation processes, as well as applications in chemical synthesis, electrochemistry, and nanotechnology.1-5 It was first reported in 2002 that imidazolium-based ILs could dissolve large amounts of cellulose that, when precipitated with water or ethanol, produced an amorphous cellulose product that could be rapidly hydrolyzed into glucose by commercial cellulase mixtures.6-10 This led to the discovery of ILs that can dissolve lignocellulose11-15 and serve to pretreat biomass prior to enzymatic hydrolysis, fractionating it into its principal components: cellulose, hemicellulose, and lignin.16-19 With increasing interest in the use of lignocellulosic biomass for the production of renewable transportation fuels, new approaches for biomass pretreatment have been of considerable interest (see Sousa et al.20 for a review). The pretreatment of biomass to enhance rates of enzymatic hydrolysis is one key to decreasing * Corresponding author. E-mail:
[email protected]. † Joint BioEnergy Institute. ‡ Biomass Science and Conversion Technology Department, Sandia National Laboratories. § Biosystems Research Department, Sandia National Laboratories.
the cost of production of monomeric sugars from biomass. Current pretreatments typically employ acids or bases at elevated temperatures and pressures to increase accessible surface area of the biomass and reduce the crystallinity of cellulose in the plant cell wall.21 IL pretreatment offers a potential solution that is able to significantly reduce cellulose crystallinity and decrease the lignin content of biomass. The effective use of ILs for biomass pretreatment requires an understanding of the molecular details of IL interactions with cellulose and lignocellulosic biomass. Despite the increasing number of known ILs that are capable of dissolving biomass,11-15 the mechanism of this dissolution process is not well understood. There have been a few theoretical and experimental studies of cellulose dissolution in ILs, including molecular dynamic studies, aimed at determining the solubility parameters of ILs, cellulose and oligomers of glucose,22 and the solvation of glucose by ILs.23 Experimentally, NMR has been used to examine the mechanisms of glucose and cellobiose dissolution, and these results indicate that the interaction is governed by specific interactions between the anions and the polysachharides, while the interactions of the cations were found to be nonspecific.24,25 The discrepancy between predicted solubility parameters and those obtained experimentally,22 and recent experimental evidence that cations are also involved in the dissolution process,26 illustrate that further research is needed to fully understand this complex interaction. Zhang et al.14
10.1021/jp9117437 2010 American Chemical Society Published on Web 03/10/2010
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J. Phys. Chem. B, Vol. 114, No. 12, 2010
Liu et al.
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