(111a) Tracking the Elementary Kinetics and Dynamic Evolution of Molecular Structures during Biomass Pyrolysis

Fast pyrolysis offers one of the most accessible ways to convert biomass into fuels and chemicals. The complexity of the feedstock and the dynamic changes in condensed phase chemical environments make it difficult to elucidate elementary kinetics and mechanisms and control the selectivity to products. These effects also manifest in macroscopic phenomena such as measurable differences in kinetics and changes in product distribution with changes in reaction temperature, feedstock composition, catalytic metal ion compositions and even dimensions of the feedstock. These complexities are ultimately dictated by molecular transformations controlled by the molecular structure and local chemical environments. Understanding the molecular processes in biomass pyrolysis is crucial for large scale and economical biomass conversion facilities.

Herein we present an ab initio-based kinetic Monte Carlo and Molecular Dynamics (kMC+MD) simulation that can model the kinetics of the fast pyrolysis and thermal reaction networks for biomass and other complex feedstocks. This approach tracks detailed atomic structural information including 3D coordinates of atoms and connectivity of chemical bonds in the feedstock molecules. It uses Stochastic Simulation Algorithm (SSA) to carry out elementary reaction steps and uses classical MD simulations to follow the dynamics of the feedstock and the reaction environment as reactions proceed. The elementary step kinetics for the kMC simulations are established from first-principle calculations.

Atomic-structural information is retained throughout the simulation, thus allowing the simulations to follow molecular transformations, the local molecular environment and the changes that result as a function of reaction conditions. As such, the simulations capture the unique kinetic manifestations that can result from the local structure of these feedstocks otherwise lost in composition-based deterministic models. This KMC+MD simulation approach is used herein to examine the pyrolysis reaction pathways of cellulose including levoglucosan and furan formation with some success in predicting kinetics and product distribution.