(334ad) Influence of Natural Catalytic Environments on the Activation of Cellulose Via Fast Pyrolysis

Fast pyrolysis is a thermal deconstruction technology that provides routes to break down macromolecules such as cellulose to smaller oxygenates that can be further upgraded to fuels and platform chemicals. Despite significant research in this area, fundamental insights into pyrolytic reaction mechanisms and the influence of the naturally present catalytic environments formed by hydroxyl groups and inorganic metals on kinetics are still lacking. This research utilizes first principles-based computations supported by experimental techniques to elucidate in detail activation (polymer degradation) mechanisms as well as the role of natural catalytic environments in enhancing activation. In relation to the natural hydroxyls, this research elucidates their influence on the kinetics of the transglycosylation reaction. It is shown that hydroxyl groups can act as proton shuttling agents and hence promote facile proton transfer reactions. Density functional theory (DFT) calculated activation energy for this route is in good agreement with activation energy measured by isotopic labeling experiments in the PHASR (Pulsed Heated Analysis of Solid Reactions) set-up (29.5 kcal mol^-1 and 26.9 ± 1.9 kcal mol^-1 respectively) thus showing that hydroxyl groups can open up seemingly catalytic pathways particularly at temperatures below 470°C. Ab initio molecular dynamics (AIMD) simulations are also used to provide insights into the unique hydrogen bonding networks formed by these hydroxyls. Specifically, the influence of reaction dynamics and entropic effects are captured using these AIMD simulations. In addition to this, influence of inorganic metals on the rates of cellulose activation is also explored. Specifically, this research elucidates the role of group II metals in disrupting hydrogen bonding networks and stabilizing charged transition states formed during cellulose activation. A detailed mechanism for cellulose activation in the presence of group II metals is presented with a DFT computed barrier that agrees well with experimental kinetics measured at 370-430°C. More generally, this research provides fundamental insights into the reaction environments that influence cellulose pyrolysis and can be used to devise strategies for engineering such environments.