Solvent molecules influence the reactions of molecular hydrogen and oxygen on palladium nanoparticles. Organic solvents activate to form reactive surface intermediates that mediate oxygen reduction through pathways distinct from reactions in pure water. Kinetic measurements and ab initio quantum chemical calculations indicate that methanol and water cocatalyze oxygen reduction by facilitating proton-electron transfer reactions. Methanol generates hydroxymethyl intermediates on palladium surfaces that efficiently transfer protons and electrons to oxygen to form hydrogen peroxide and formaldehyde. Formaldehyde subsequently oxidizes hydrogen to regenerate hydroxymethyl. Water, on the other hand, heterolytically oxidizes hydrogen to produce hydronium ions and electrons that reduce oxygen. These findings suggest that reactions of solvent molecules at solid-liquid interfaces can generate redox mediators in situ and provide opportunities to substantially increase rates and selectivities for catalytic reactions.

Fast pyrolysis is a thermochemical process to fragment large biopolymers such as cellulose to chemical intermediates which can be refined to renewable fuels and chemicals. Levoglucosan (LGA), a six-carbon oxygenate, is the most abundant primary product from cellulose pyrolysis with LGA yields reported over a wide range of 5–80 percent carbon (%C). In this study, the variation of the observed yield of LGA from cellulose pyrolysis was experimentally investigated. Cellulose pyrolysis experiments were conducted in two different reactors: the Frontier micropyrolyzer (2020-iS), and the pulse heated analysis of solid reactions (PHASR) system. The reactor configuration and experimental conditions including cellulose sample size were found to have a significant effect on the yield of LGA. Four different hypotheses were proposed and tested to evaluate the relationship of cellulose sample size and the observed LGA yield including (a) thermal promotion of LGA formation, (b) the crystallinity of cellulose samples, (c) secondary and vapor-phase reactions of LGA, and (d) the catalytic effect of melt-phase hydroxyl groups. Co-pyrolysis experiments of cellulose and fructose in the PHASR reactor presented indirect experimental evidence of previously postulated catalytic effects of hydroxyl groups in glycosidic bond cleavage for LGA formation in transport-limited reactor systems.

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 13C1 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.

Naturally occurring metals, such as calcium, catalytically activate the intermonomer β-glycosidic bonds in long chains of cellulose, initiating reactions with volatile oxygenates for renewable applications. In this work, the millisecond kinetics of calcium-catalyzed reactions were measured via the method of the pulse-heated analysis of solid and surface reactions (PHASR) at high temperatures (370–430 °C) to reveal accelerated glycosidic ether scission with a second-order rate dependence on the Ca 2+ ions. First-principles density functional theory (DFT) calculations were used to identify stable binding configurations for two Ca 2+ ions that demonstrated accelerated transglycosylation kinetics, with an apparent activation barrier of 50 kcal mol –1 for a cooperative calcium-catalyzed cycle. The agreement of the mechanism with calcium cooperativity to the experimental barrier (48.7 ± 2.8 kcal mol –1 ) suggests that calcium enhances the reactivity through a primary role of stabilizing charged transition states and a secondary role of disrupting native H-bonding.

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. File list (2) download file view on ChemRxiv ChemRxiv_Narrative_Vineet_AlphaBeta_ver_05.pdf (0.92 MiB) download file view on ChemRxiv Vineet_theory_GBactivation_SI_ver_02.pdf (576.67 KiB)