(180c) Revealing Molecular-Level Interactions and Kinetics of Cellulose Decomposition during Its Co-Pyrolysis with Lignin

Production of renewable energy and chemicals from lignocellulosic biomass is projected to grow significantly in the coming decades with increased global concerns on the energy shortage and detrimental environmental consequences caused by ever-increasing fossil fuel consumption. Fast pyrolysis is a promising thermochemical method to convert biomass feedstocks into biofuels or biomass-derived chemicals in the absence of oxygen at 400—1000 °C, but its product distribution is inherently non-selective due to its diverse compositions and complex reaction networks. Binary interactions between the major constituents of biomass during co-pyrolysis are still actively debated, with contradictory results reported on cellulose-lignin interactions by different researchers. Despite the experimental efforts, for instance, the molecular-level interactions of cellulose and lignin are unknown. This work focuses on elucidating the reaction pathways and kinetics of cellulose decomposition during its co-pyrolysis with lignin. Cellulose activation and subsequent depolymerization were investigated in the presence of lignin using dispersion-corrected density functional theory calculations (DFT-D3). A comparison of the neat cellulose and cellulose-lignin pyrolysis kinetics reveals that lignin functional groups containing hydrogen bond acceptors, such as ethers, impede cellulose activation, while lignin functional groups solely bearing hydrogen bond donors, such as alcohols, promote the concerted scission of β-1,4 glycosidic bond. Our calculations show that monolignol models such as coniferyl alcohol and sinapyl alcohol resulted in an increase in the activation energy of cellulose activation by approximately 5 kcal/mol, compared to that of neat cellulose activation of 47.0 kcal/mol, while p-coumaryl alcohol stabilizes the transition state, lowering the activation energy by 4 kcal/mol. The entangled binding nature of lignin suggests that different functional groups of lignin can result in a catalytic or inhibitory effect on cellulose pyrolysis. This new molecular-level understanding of the co-pyrolysis behavior provides insight into the functional group effects on the reaction mechanisms and kinetic modeling of biomass pyrolysis chemistry.