(556c) Theoretical Elucidation of the Molecular Behaviours of Key Compounds during Biomass Pyrolysis

Biomass pyrolysis is an efficient way to transform raw biomass or organic waste materials into useable biofuels and/or valuable chemicals. The production process can be described as biomass components decompose at various depolymerization stages based on their stability and operating temperature: firstly hemicellulose, followed by cellulose, and lignin decomposition at the final stage. During this decomposition sequence, thousands of compounds/intermediates (e.g., anhydro-sugars, aromatics, phenols) form, react, remain or disappear in the system. Despite the investigation history on the production of those compounds at the macro scales by both experiments and modellings, the fundamental atomic/molecular level investigations in this area have not been extensively carried out to date.

Levoglucosan (1,6-anhydro-β-D-glucopyranose) is representative of one of these compounds. It is one important primary product during cellulose pyrolysis either as an intermediate or as a product. It is initially formed from cellulose decomposition and further converts into low-molecular-weight compounds during pyrolysis secondary reactions. It remains one of the main compounds of raw bio-oil, comprising up to 78 wt%.

Within this talk, the fundamental investigation on the mechanism and kinetic modelling of the production of levoglucosan from lignocellulosic biomass will be presented. Three available mechanisms for levoglucosan formation have been studied theoretically by performing density functional theory based calculations. These mechanisms are free-radical mechanism; glucose intermediate mechanism; and levoglucosan chain-end mechanism. Specifically, the molecular behaviour of levoglucosan has elucidated by revealing 14 reaction pathways, 26 elemental reaction steps, and the involved around 60 compounds act as intermediates, transition states, or products. By comparing with the activation energy obtained from the experimental results, it was concluded that free-radical mechanism has the highest energy barrier during the three levoglucosan formation mechanisms, glucose intermediate mechanism has lower energy barrier than free-radical mechanism, and levoglucosan chain-end mechanism is the most reasonable pathway because of the lowest energy barrier. The variational transition state rate constants for every elementary reaction and every pathway were calculated. The first-order Arrhenius expressions for these elementary reactions and pathways were suggested.

As an intermediate during cellulose pyrolysis, levoglucosan will be easily decomposed as part of secondary reactions of tar. The thermal decomposition mechanism of levoglucosan was also studied using density functional theory methods. The decomposition included direct C-O bond breaking, direct C-C bond breaking, and dehydration. The properties of the reactants, transition states, intermediates, and products for every elementary reaction were obtained. It was concluded that C-O bond breaking is easier than C-C bond breaking due to a lower activation energy and a higher released energy. During the 6 levoglucosan dehydration pathways, one water molecule which composed of a hydrogen atom from C3 and a hydroxyl group from C2 is the preferred pathway due to a lower activation energy and higher product stability.

Except for levoglucosan, the formation pathways of other compounds were also investigated. It was found that formaldehyde can be formed from anhydroglucose which undergoes a hydrogen-donor reaction, the anhydroglucose can be obtained from a cellulose hemolytic reaction during high-temperature steam gasification of the biomass process. At a pressure of 1 atm, levoglucosan can be formed at all of the temperatures (450-750 K) considered in this simulation, whereas formaldehyde can be formed only when the temperature is higher than 475 K. Moreover, the energy barrier of levoglucosan formation is lower than that of formaldehyde. It was also found that 1-pentene-3,4-dione, acetaldehyde, 2,3-dihydroxypropanal, and propanedialdehyde can be formed from the C-O bond breaking decomposition reactions. 1,2-Dihydroxyethene and hydroxyacetic acid vinyl ester can be formed from the C-C bond breaking decomposition reactions.

This research provides thermodynamic assessment of the available routes for the production of key compounds, identifies key challenges and future trends for second-generation bio-chemicals. It also confirms that Quantum Mechanics based simulation can reveal fundamental phenomena, which are difficult to be explored from traditional experimental techniques, and can be used to guide the experimental design and industrial application.