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Levoglucosan Formation from Crystalline Cellulose: Importance of a Hydrogen Bonding Network in the Reaction

Authors

  • Dr. Takashi Hosoya,

    Corresponding author
    1. Department of Chemistry, University of Natural Resources and Life Sciences, Vienna, Muthgasse 18, 1190 Vienna (Austria)
    • Department of Chemistry, University of Natural Resources and Life Sciences, Vienna, Muthgasse 18, 1190 Vienna (Austria)

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  • Prof. Dr. Shigeyoshi Sakaki

    1. Fukui Institute for Fundamental Chemistry Kyoto University, Takano-nishihiraki-cho 34-4, Kyoto 606-8103 (Japan)
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Abstract

Levoglucosan (1,6-anhydro-β-D-glucopyranose) formation by the thermal degradation of native cellulose was investigated by MP4(SDQ)//DFT(B3LYP) and DFT(M06-2X)//DFT(B3LYP) level computations. The computational results of dimer models lead to the conclusion that the degradation occurs by a concerted mechanism similar to the degradation of methyl β-D-glucoside reported in our previous study. One-chain models of glucose hexamer, in which the interchain hydrogen bonds of real cellulose crystals are absent, do not exhibit the correct reaction behavior of levoglucosan formation; for instance, the activation enthalpy (Ea=≈38 kcal mol−1) is considerably underestimated compared to the experimental value (48–60 kcal mol−1). This problem is solved with the use of two-chain models that contain interchain hydrogen bonds. The theoretical study of this model clearly shows that the degradation of the internal glucosyl residue leads to the formation of a levoglucosan precursor at the chain end and levoglucosan is selectively formed from this levoglucosan end. The calculated Ea (56–62 kcal mol−1) agrees well with the experimental value. The computational results of three-chain models indicate that this degradation occurs selectively on the crystalline surface. All these computational results provide a comprehensive understanding of several experimental facts, the mechanisms of which have not yet been elucidated.

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