The Influence of High Temperature Pyrolysis Melt and Lignin on Cellulose  Pyrolysis Chemistry: A First-principles based Investigation
Creators: Padmanathan, Arul Mozhi Devan
Publisher: name: University of Alberta Library
Year: 2023

<p>Lignocellulosic biomass pyrolysis for biofuel production shows potential but is challenged with issues of bio-oil instability and low yield. To address these challenges, a bottom-up approach integrating reaction chemistry and transport within biomass particle during pyrolysis is crucial for predicting global performance and engineering parameters. At high pyrolytic temperatures, cellulose undergoes an amorphous transformation into an active-cellulose/melt-phase, making it essential to investigate the influence of this condensed phase environment on cellulose reaction kinetics. Moreover, the pyrolysis product yields from native biomass show stark differences from the pyrolysis of physically mixed synthetic biomass highlighting the role played by lignin-carbohydrate complex (LCC) linkages. Hence, this thesis focuses on investigating the chemistry of the pyrolytic decomposition of cellulose and the influence of condensed phase environment, lignin and LCC linkages on its decomposition. <br>
A novel computational strategy is developed to limit computational cost, employing a hybrid approach that combines density functional theory (DFT) and molecular dynamics (MD) methods. Calculations using this novel strategy reveal two distinct cellulose decomposition regimes transitioning at 900 K, in line with millisecond-scale kinetic experiments. At high temperatures, the reduction in hydrogen bonding and the shift in hydroxymethyl orientations result in a lowered enthalpic barrier within the melt/active phase. As temperature increases, the melt-phase incurs an entropic penalty due to increased degree of freedom exhibited by cellulose chains, reducing the free-energy barrier and leading to an entropy-driven decomposition. Such entropic reductions are significantly less pronounced in the gas phase, indicating that the condensed phase environment further enhances entropic losses. Furthermore, in native biomass, despite clear evidence for the impact of lignin on cellulose decomposition, its mechanism remains poorly understood. To investigate such condensed phase influence of lignin and LCC, two different environments are modeled: one with covalent linkages between lignin and cellulose, and one without such linkages, under pyrolysis conditions. <br>
The presence of lignin and covalent interactions with cellobiose within the lignin-carbohydrate complex (LCC) and lignin-rich melt-phase have been found to influence the reaction energetics differently. In the LCC melt-phase, there is a promotion of cellulose activation, leading to a significant 107 kJ/mol reduction in the free energy barrier between 100K and 1200K for transglycosylation. This creates two distinct reaction regimes, resembling the behavior observed in pure cellulose. On the other hand, in the lignin-rich melt-phase, the condensed phase environment has no notable impact. Despite the different thermal responses, all three local reaction environments show that the disruption of the hydrogen bonding network and subsequent conformational flexibility in the hydroxymethyl group orientation directly affects the thermal shift in the free-energy barriers for cellobiose activation. Furthermore, the trend in the calculated free energy barriers for cellobiose activation in pure cellobiose and the lignin-rich melt-phase aligns with experimental millisecond-scale kinetics for the pyrolysis of pure cellulose and Loblolly pine. <br>
To also account for the influence of covalent bonding between lignin and cellulose, we have conducted first-principles DFT calculations to investigate the breakdown of cellulose cross-linked with lignin in lignin-carbohydrate complexes (LCCs). The LCC models used in the study incorporate β-O-4 benzyl ether linkages and are employed to analyze the energetics associated with three mechanisms (transglycosylation, ring contraction, and ring opening) that produce crucial biomass pyrolysis products, namely levoglucosan (LGA), furans, and glycolaldehyde. The examination of activation barriers and reaction energies reveals significant differences induced by LCC linkages on cellulose decomposition kinetics and thermochemistry. Specifically, cross-linked cellobiose in LCC exhibits higher activation barriers (2X) and reaction energies (3-4X) compared to pure cellobiose. The higher reaction energy observed for glycolaldehyde, in contrast to LGA, highlights its preferential formation at higher temperatures due to its more endergonic nature. This reduction in LGA at higher temperatures finds support in the product distribution observed in thin-film bagasse pyrolysis. Moreover, comparing relative experimental yields with the calculated reaction barriers provides evidence suggesting change in the reaction mechanism between cross-linked cellulose in LCC and pure cellulose. This inference is further supported by HOMO-LUMO analysis, which reveals a shift in HOMO orbitals from cellulose to the lignin moiety, suggesting the possibility of inter-moiety mechanisms. In summary, the study sheds light on the intricate effects of LCC linkages on cellulose degradation, highlighting both kinetic and thermochemical alterations in the pyrolysis products formation.</p>