Date of Award
8-16-2024
Document Type
Open Access Dissertation
Department
Chemistry and Biochemistry
First Advisor
Aaron Vannucci
Abstract
Lignin is a highly aromatic, branched polymer and could be utilized as a renewable source of liquid fuel. The advent of “lignin-first” depolymerization processes have allowed for the high-yield isolation of oxygenated aromatic monomers from lignin. These oxygenated aromatic compounds are lower in energy and stability than their deoxygenated counterparts. Selective deoxygenation of these compounds would result in stable, higher value liquid fuels. Using molecular catalysts, selective deoxygenation, or hydrodeoxygenation (HDO), of lignin-derived compounds in the presence of hydrogen is possible.
Immobilization of these molecular catalysts on metal oxide surfaces eliminates the possibility of bimolecular decomposition and allows for easier separations. These so-called hybrid catalysts are more industrially friendly while retaining the high activity and selectivity of molecular catalysts than that of molecular catalysts on their own. Molecular catalyst stability tends to be quite low at elevated temperatures. This instability makes the activation of stronger C-O bonds found in lignin-derived molecules difficult. This work aims to investigate the stability of molecular HDO catalysts to develop more stable and active molecular catalysts and establish experimental methods to study said catalysts. Molecular catalysts are known to decompose into metallic nanoparticles at elevated temperatures. These metallic nanoparticles can often be catalytically active.
Chapter 2 details experimental methods for determining the active catalytic species present in a typical batch HDO system. A 2,6-bis(4-isopropyl-2-oxazolin-2-yl)pyridine palladium(II) molecular catalyst was chosen as it should readily decompose under moderate HDO conditions for the purpose of developing experimental methods of determining the active species in a given reaction. The molecular catalyst was found to be moderately active for the HDO of benzyl alcohol at room temperature. Full conversion of benzyl alcohol to toluene occurred at elevated temperatures due to decomposition of the molecular catalyst into an active metallic nanoparticle catalyst. The metallic nanoparticles were reused for several cycles and catalytic activity was not retained. Post reaction mixtures were examined for catalytically active leeched metal species. It was found that post reaction solutions contained catalytically active species leeched from the metallic nanoparticle species.
The aim of Chapter 3 is to explore the HDO activity of a hybrid chloro(2,6-bis(1-methylbenzimidazolyl)pyridine-4’-aminopropyltrisiloxane)Palladium(II) chloride catalyst for the conversion of lignin-derived compounds. This catalyst was able to selectively deoxygenate benzylic oxygenates at atmospheric pressures of H2 with faster kinetics and lower catalyst loadings than previously reported palladium HDO catalysts. Pre-reaction and post-reaction characterization do not show the presence of metallic nanoparticles. These results indicate that catalytic activity can be attributed to a palladium molecular catalyst immobilized on a solid silica support.
Chapter 4 details a study into the potential HDO activity of ruthenium molecular catalysts. Ruthenium was chosen as their organometallic species are well studied and tend to exhibit high stability under a variety of conditions. Ruthenium organometallic compounds are also known to perform dehydrogenation reactions and should be able to perform the reverse reaction under proper conditions. Three ruthenium compounds were chosen: Ru(Mebimpy)Cl3, and [Ru(Mebimpy)(bpy)Cl]Cl, and Ru(tpy)Cl3. The activity of these catalysts was poor compared to that of previously reported palladium HDO catalysts, however, a number of insights were discovered in regard to ruthenium molecular catalyst HDO activity. The number of available coordination sites was found to be key in achieving higher catalytic activity. Dehydrogenation and HDO reaction pathways are both possible using these ruthenium complexes and, as competitive processes, inhibit the others’ activity. Reactivity achieved with phenol, although low, is the first seen with molecular complexes.
Rights
© 2024, Jacob G. Tillou
Recommended Citation
Tillou, J. G.(2024). Molecular and Hybrid Catalysts for Selective Hydrodeoxygenation of Lignin Model Compounds. (Doctoral dissertation). Retrieved from https://scholarcommons.sc.edu/etd/7717