(589c) Discerning and Modeling Reaction Mechanisms in the Cellulose Pyrolysis Reaction Network

Biomass fast pyrolysis is a promising technology for the thermal conversion of biomass to liquid fuels. With less-severe operating temperatures than gasification, fast pyrolysis produces a primarily liquid product, which requires less-costly equipment for processing and storage than a vapor product. Currently, improvements to make this technology more economically attractive are impeded by a lack of understanding of the reaction chemistry, which has been difficult to ascertain due to the complex reaction network accessible at the temperatures employed, typically near 500 °C, and the short timescales on the order of seconds. Without this information, we cannot predict how changes in reaction conditions and feeds will affect product yield and composition, knowledge of which are crucial for process design and optimization. To overcome this obstacle, we employ density functional theory to compare competing reaction mechanism proposals and examine fleeting transition states, knowledge of which can aid in rational catalyst design. Specifically, we have focused on reactions central to cellulose fast pyrolysis, including evaluation of several competing theories for the formation of the main product of pure cellulose pyrolysis, levoglucosan, and uncovered a new mechanism that yields results consistent with experiment. We also evaluated several key decomposition reactions of glucose, the cellulose monomer and postulated key intermediate, and how they are perturbed by alkali metal ions naturally present in biomass. We incorporated the results of these studies in the first mechanistic model of cellulose pyrolysis, which offers predictions of detailed product speciation that closely match experimental results.