Activation of Cellulose via Cooperative Hydroxyl-Catalyzed Transglycosylation of Glycosidic Bonds

The thermal activation of cellulose by initial glycosidic bond cleavage determines the overall rate of conversion to organic products for energy applications. Here, the kinetics of ether scission by transglycosylation of β-1,4-glycosidic bonds was measured using the "pulse-heated analysis of solid reactions" (PHASR) method from 400 to 500 °C. Levoglucosan (LGA) formation from cellulose was temporally resolved over the full extent of conversion, which was interpreted via a coupled reactant-product evolution model to determine an apparent barrier of LGA formation of 27.9 kcal mol ^-1 . In parallel, LGA formation from the glucose monomer of cellobiosan was measured at temperatures between 380 and 430 °C by isotopically labeling the ¹³ C ₁ carbon; an apparent activation energy of LGA formation was measured as 26.9 ± 1.9 kcal mol ^-1 . The unusually low activation barrier for LGA formation at lower temperature is in agreement with previous PHASR studies for cellulose breakdown and is indicative of catalytic rather than thermal C-O bond activation. A catalytic mechanism was proposed wherein vicinal hydroxyl groups from neighboring cellulose sheets promote transglycosidic C-O bond activation. First-principle density functional theory (DFT) calculations showed that these vicinal hydroxyl groups cooperatively act to create an environment that (a) stabilizes charged transition states and (b) aids in proton transfer, thus leading to reduced activation barriers for transglycosylation. Models incorporating intrasheet H bonding of cellulose were also used to establish their influence on kinetics. The calculated apparent barrier (29.5 kcal mol ^-1 ) agreed well with the experimental apparent activation energy (26.9 ± 1.9 kcal mol ^-1 ) and establishes the dominant mode for cellulose activation and subsequent levoglucosan formation at lower temperatures (<467 °C) as site-specific, vicinal hydroxyl-catalyzed transglycosylation.