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

preprint
submitted on 12.05.2020 and posted on 14.05.2020 by Paul Dauenhauer, Vineet Maliekkal, Matthew Neurock
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 an 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 52.4 kcal/mol, which is similar to the barriers reported for non-catalytic transglycosylation mechanism (~50 kcal/mol). 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 (~32-37 kcal mol-1) in agreement with previous experiments.

Funding

We acknowledge support from the U.S. Department of Energy, Office of Basic Energy Science Catalysis (DE-SC0016346).

History

Email Address of Submitting Author

hauer@umn.edu

Institution

University of Minnesota

Country

USA

ORCID For Submitting Author

0000-0001-5810-1953

Declaration of Conflict of Interest

No conflict of interest

Version Notes

Original submission - version 1.0

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