Glycosidic C-O Bond Activation in Cellulose Pyrolysis: Alpha Versus Beta and Condensed Phase Hydroxyl-Catalytic Scission

Mechanistic insights into glycosidic bond activation in cellulose pyrolysis were obtained via first-principles density functional theory calculations that explain the peculiar similarity in kinetics for different stereochemical glycosidic bonds (β vs α) and establish the role of the three-dimensional hydroxyl environment around the reaction center in activation dynamics. The reported activating mechanism of the α-isomer was shown to require the initial formation of a transient C1-O2-C2 epoxide that subsequently undergoes transformation to levoglucosan. Density functional theory results from maltose, a model compound for the α-isomer, show that the intramolecular C2 hydroxyl group favorably interacts with lone pair electrons on the ether oxygen atom of an α-glycosidic bond in a manner similar to the hydroxymethyl (C6 hydroxyl) group interacting with the lone pair electrons on the ether oxygen atom of a β glycosidic bond. This mechanism has an activation energy of 219 kJ mol-1, which is similar to the barriers reported for noncatalytic transglycosylation mechanism (∼209 kJ mol-1) and in good agreement with experimentally measured barriers for α-cyclodextrin conversion at high temperatures. The results help explain the lack of sensitivity of depolymerization kinetics to glycosidic bond stereochemistry. Subsequent constrained ab initio molecular dynamics (AIMD) simulations revealed that vicinal hydroxyl groups in the condensed environment of a reacting carbohydrate melt anchor transition states via two-to-three hydrogen bonds and lead to lower free energy barriers (∼134-155 kJ mol-1) in agreement with previous experiments.