(383f) Investigating Primary Decomposition Pathways of Cellulose during the Pyrolysis Process Using First Principles Methods

Pyrolysis of lignocellulosic biomass is a promising method to convert it to fuels. However, development of the pyrolysis process to improve the quality and yield of bio-oil is hindered by the limited knowledge of the underlying chemistry and transport. Though experimental studies can explain the overall kinetics of biomass decomposition during pyrolysis, they fail to provide the fundamental understanding of reaction mechanisms, pathways and energetics. This is also crucial to create a ‘building up’ effect, enabling the integration of chemical mechanisms in particle level models and engineering the interplay between chemistry and transport to optimize the product, bio-oil. Hence in this study, the temperature-variant decomposition of cellobiose, a model compound for cellulose, during pyrolysis is investigated using a novel condensed phase transition state (TS) search method (ConTS), benchmarked with Car-Parrinello-molecular-dynamics-Metadynamics. ConTS integrates force-field molecular dynamics with Density-Functional-Theory (DFT) TS-search calculations to include the effect of condensed phase and largely reduces the computational cost. Two primary cellobiose decomposition pathways were hypothesized – at temperatures < 470°C, crystalline cellobiose undergoes an amorphous phase transformation before decomposition; while at higher temperatures, it undergoes direct decomposition. Free-energy analysis of these two pathways was performed using thermodynamic integration method algorithms. In addition, ConTS calculations revealed that the increased inter-sheet hydrogen bonding in the “amorphous” phase stabilizes the transition state and thereby decreasing the activation barrier for the cleavage of the glycosidic bond via transglycosylation. This established relationship between the increased hydrogen bonding and the reduction in activation barrier was previously unexplored. Investigation of glycosidic bond cleavage via ring contraction revealed that in addition to the difference in kinetics and energetics with temperature, cellulose decomposition shows competitive reaction mechanisms for bond cleavage. The temperature-dependent decomposition pathways and corresponding energetics arises as a result of the change in the molecular arrangement in the condensed phase pyrolysis environment.