Date of Award
12-20-2024
Degree Name
Doctor of Philosophy (PhD)
Degree Type
Dissertation
Abstract
Lignin is a complex, random, and heterogeneous biopolymer in lignocellulosic biomass. It has significant potential for conversion into valuable chemicals and fuels. However, its natural recalcitrant structure has made selective depolymerization challenging. This dissertation presents a detailed investigation into the mechanisms of lignin disassembly and structural evolution under various catalytic and solvent conditions. The research spans three key studies. In Chapter 2, operando magic-angle spinning (MAS) solid-state nuclear magnetic resonance (NMR) spectroscopy was employed to study reaction pathways and kinetics of lignin model polymers during catalytic hydrogenolysis. We used a Ni-Al2O3 catalyst in methanol under high temperature and pressure, and we revealed that multiple parallel pathways, including α-dehydroxylation and methoxylation. This chapter also shows the rates of polymer chain scission and the stabilization of monomeric products at different temperatures. Chapter 3 investigates the structural evolution of lignin during catalytic disassembly using in-situ small-angle neutron scattering (SANS). Lignin was studied with a copper-containing porous metal oxide (CuPMO) catalyst in methanol at 250°C. The findings show that lignin undergoes significant conformational changes, transitioning from a rigid, stretched state to a flexible, globular structure at reaction conditions. The SANS data reveals two major processes: 1) the disassembly of large lignin aggregates and 2) the condensation of smaller lignin particles. This chapter highlights the importance of in-situ techniques in capturing the real-time structural dynamics of lignin during depolymerization. In Chapter 4, a combination of SANS and molecular dynamics (MD) simulations was used to explore the behavior of lignin aggregates in γ-valerolactone (GVL) solvent systems. The study used lignin extracted at different temperatures (100°C and 120°C, noted as L100 and L120 respectively) and examined its interactions with GVL under varying temperature conditions. Results showed that lignin solvation and aggregation are strongly dependent on its molecular structure, with L120 lignin demonstrating greater solvent affinity and structural flexibility compared to L100 lignin. MD simulations confirmed the enhanced solvent-lignin interactions in GVL, mostly due to the presence of methoxy groups in the L120 lignin oligomers. Overall, these studies provide a comprehensive understanding of lignin depolymerization mechanisms and structural behavior in different catalytic and solvent environments. The insights gained from operando NMR, in-situ SANS, and MD simulations contribute to the more efficient processes for lignin valorization in biorefinery applications.
Language
English (en)
Chair
Marcus Foston
Committee Members
Daniel Giammar; Joshua Yuan; Kimberly Parker; Manjula Senanayake