Levoglucosan formation from crystalline cellulose: importance of a hydrogen bonding network in the reaction.
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.
Department of Chemistry, University of Natural Resources and Life Sciences, Vienna, Muthgasse 18, 1190 Vienna (Austria). takashi.hosoya@boku.ac.at.
Takashi
T
Hosoya
Shigeyoshi
S
Sakaki
D002482:Cellulose / Q000737:chemistry*; D005947:Glucose / Q000031:analogs & derivatives* / Q000737:chemistry; D006358:Hot Temperature; D006860:Hydrogen Bonding; D008956:Models, Chemical; D008958:Models, Molecular
C014989:1,6-anhydro-beta-glucopyranose; D002482:Cellulose; D005947:Glucose
biomass; chain structures; conformation analysis; density functional calculations; hydrogen bonds