(34d) Energetics of Condensed-Phase Biomass Pyrolysis Chemistry: A Novel Computational Approach and Its Benchmarking Using Ab-Initio Molecular Dynamics

Optimization of the pyrolysis process to improve the quality and yield of bio-oil for commercialization is hindered by the limited knowledge of its underlying chemistry. Experimental studies can explain the overall kinetics of biomass decomposition but fail to provide the fundamental understanding of reaction pathways and energetics. Hence, in this work, primary cellulose (major biomass component) decomposition reactions are modeled at pyrolysis temperatures using a combination of Density Functional Theory (DFT) transition state (TS) search and molecular mechanics based thermodynamic integration (TI) methods. DFT TS-search is first performed in gas phase and the corresponding free-energy barrier is corrected using TI calculations to include condensed phase effects. The novel technique is used to overcome the exceptionally high computational cost associated with ab initio molecular dynamics methods in capturing cellulose chemistry in the temperature-varying pyrolysis environment. It is then benchmarked using Car-Parrinello molecular dynamics accelerated by metadynamics (CPMD-metadynamics). Metadynamics acceleration is performed along two collective variables to simulate glycosidic bond cleavage. Interactions in the condensed phase are captured explicitly with four-unit cells of cellobiose repeating periodically in three dimensions. It was observed that condensed phase free energy barriers for glycosidic bond cleavage drop drastically above 900K making two reaction regimes – high barrier at low temperatures and low barrier at high temperatures. With increasing temperature, the hydrogen bonding network in cellulose is disrupted leading to the destabilization of the reactant and reduction in the barrier. This drop in free energy barrier is substantiated by CPMD-metadynamics calculations performed at 500K and 1200K. Structural comparison of key intermediates and their interactions with neighboring molecules in the condensed phase indicate destabilization of the reactant and no preferential stabilization of the TS. This highlights the role of condensed phase in cellulose decomposition and validates the accurate representation of temperature appropriate reaction environment in the proposed approach.