The Mechanocatalytic Depolymerization of Lignin and Lignin Model Compounds
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Phillips, Erin Victoria
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Abstract
Lignocellulosic biomass is a promising bio-based renewable resource that has the potential to replace traditional methods of fuel and chemical production. Lignin constitutes approximately one-third of the lignocellulose structure, and is primarily composed of aromatic units, but is often overlooked due to its amorphous complexity. The valorization of lignin presents many challenges, due to its recalcitrant nature, but if properly depolymerized, could serve as a sustainable feedstock for various chemical sectors such as the plastic, pharmaceutical, disinfectant and bio-fuel industries. To study the lignin structure in isolation, model compounds are often used to determine successful approaches to targeted cleavage of the interconnecting bonds found within the lignin structure. Mechanocatalysis is a promising alternative approach to this depolymerization and is a green chemistry alternative that accelerates reactions under ambient conditions. External sources of heat and pressure are not necessary to drive mechanocatalytic reactions, and the technique is environmentally friendly. Fundamental understanding of how mechanocatalytic reactions are driven is still under investigation, but results have demonstrated that it is an efficient, solvent free alternative. The driving force for mechanochemical reactions is thought to be a culmination of various phenomena including hotspot formation, increased solid-solid mixing between feedstock and catalyst, the consistent formation of transient and defect sites, and triboelectric effects.
The aim of this thesis is centered around gaining a greater understanding of the use of heterogenous catalysts for lignin depolymerization, and how mechanochemical systems can be applied as a lignin valorization technique. Ether bond cleavage was the primary focus of this work, wherein various forms of palladium, platinum and ruthenium supported on carbon, silica and alumina were used to promote the hydrogenolysis of ether linkages found in the lignin structure. Piezoelectric materials including barium titanate, sodium niobate and bismuth ferrite were also investigated. The mechanocatalytic apparatus was adapted for gas flow, so hydrogen could be provided to the reaction environment throughout the milling process for both exploratory lignin experiments, in addition to model dimer studies. Achieving effective valorization has focused on a deeper understanding of catalyst behaviors and capabilities, and how catalytic properties play a role in hydrogenolysis. To exploit the favorability of successful catalysts with this work, properties such as the intrinsic ability to form hydrides, support inertness, and resistance to deformation were sought after, to ensure high levels of reactivity over extended milling periods. Recognizing mechanochemical limitations in addition to inherent obstacles associated with lignin valorization, such as recalcitrance, was important to identifying which catalysts were likely to be successful. The novel understanding developed throughout the entirety of this thesis serves as important foundational groundwork for the study of lignin hydrogenolysis using heterogeneous catalysts.
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2025-04-29
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Dissertation