(590f) Advancing Lignin Valorization: Elucidating Thermodynamic and Kinetic Properties through an Integrated Density Functional Theory and Kinetic Monte Carlo Approach

Lignin, a complex aromatic polymer found in plant cell walls, is a highly attractive feedstock for producing renewable chemicals and fuels due to its abundant availability and significant potential for valorization^[1]. As the second most abundant polymer on Earth after cellulose, lignin plays an important role in plant growth, development, and defense against pathogens^[2]. Nevertheless, the complex structure of lignin currently poses a substantial challenge for the development of sustainable materials and processes, as its resistance to degradation complicates its utilization. Therefore, understanding the de/repolymerization behaviors of lignin is essential for devising more efficient and sustainable valorization processes, which could potentially reduce our dependence on fossil fuels^[3].

In this study, we employed a multifaceted approach to investigate the de/repolymerization of lignin by integrating density functional theory (DFT) method and kinetic Monte Carlo (kMC) simulations, which allowed us to consider various factors affecting the de/repolymerization behavior. Specifically, DFT calculations enabled us to predict structural changes, thermodynamic properties, and reaction pathways of lignin de/repolymerization at the atomic scale^[4-5]. However, DFT alone is insufficient for modeling the kinetics of lignin systems (i.e., lignin monomer yield and chain length distribution) at the microscopic level. This is where kMC simulations prove invaluable, as they facilitate a deeper exploration of the kinetics of lignin de/repolymerization reactions by simulating the time evolution of complex systems^[6].

To accurately interpret the results of kMC simulations, it is necessary to integrate the DFT method, as it provides critical information on lignin de/repolymerization, such as activation energy barriers and their variations according to lignin size and changing structural conformations. This information can then guide the interpretation of kMC simulations, offering insights into the time-dependent concentration of lignin fragments and the distribution of lignin sizes.

By employing integrated DFT and kMC simulations, we aimed to obtain a more accurate interpretation of lignin de/repolymerization reactions under various conditions, including temperature and lignin size. Specifically, we examined the temperature dependency of activation energy barriers required for de/polymerization reactions and how the size and structural conformation of lignin can influence this dependence. We believe that our comprehensive, multifaceted approach provides valuable insights into the thermodynamic and kinetic properties of lignin systems, which will ultimately aid in the development of valuable products (i.e., carbon fiber, phenolic compounds, multifunctional hydrocarbon and syngas) more effectively.

References

[1] Zeng, Y., Himmel, M. E., and Ding, S-Y. Visualizing chemical functionality in plant cell walls. Biotechnol Biofuels, 2017, 10, 263.

[2] Satteler, S. E, and Funnell-Harris, D. L. Modifying lignin to improve bioenergy feedstocks: strengthening the barrier against pathogens? Front. Plant Sci, 2013, 4.

[3] Iram, A, Berenjian, A, and Demirci, A. A Review on the Utilization of Lignin as a Fermentation Substrate to Produce Lignin-Modifying Enzymes and Other Value-Added Products. Molecules. 2021, 26(10), 2960.

[4] Mu, X., Han, Z., Liu, C., and Zhang, D., Mechanistic Insights into Formaldehyde-Blocked Lignin Condensation: A DFT Study. J. Phys. Chem. C, 2019, 123, 8640-8648.

[5] Azad, T., Auad, M. L, Elder, T., and Adamczyk, A. J., Toward Native Hardwood Lignin Pyrolysis: Insights into Reaction Energetics from Density Functional Theory. Energy Fuels, 2023, 37, 401-423.

[6] Eswaran, S. C. D., Subramaniam. S., Sanyal, U., Rallo, R, and Zhang, X., Molecular structural dataset of lignin macromolecule elucidating experimental structural compositions. Nature, 2022, 9, 647.