Date of Award
Doctor of Philosophy (PhD)
J. Will. Medlin
Daniel K. Schwartz
In this thesis, we investigate the modification of Pd/Al2O3 catalyst using thiolate and phosphonate self-assembled monolayers (SAMs) for two types of reactions in liquid-phase environments: selective oxidation of α, β-unsaturated alcohols and vanillin hydrodeoxygenation. Through the study, we aim to gain a better understanding of the liquid-solid interface that governs the catalytic activity.
The selective oxidation of a linear unsaturated alcohol trans-2-hexen-1-ol (HOL) was studied on Pd/Al2O3 catalysts in heptane. Alkanethiolate self-assembled monolayers (SAMs) were applied to modify Pd/Al2O3 catalysts; the sulfur content on the Pd surface was tuned by applying different tail structures of the thiolate modifier. The modification successfully limited the deactivation of Pd/Al2O3 during HOL oxidation while maintaining the desired selectivity to the corresponding aldehyde trans-2-hexen-1-al (HAL). Reaction studies combined with infrared spectroscopy analysis were used to investigate the role of the thiolate SAMs in controlling the formation of the surface poisoning species and preventing catalyst deactivation.
Another type of industrially important reaction, hydrodeoxygenation (HDO), was studied on Pd/Al2O3 catalysts using vanillin as a probe reactant. The reaction kinetics was first explored in an ethanol solvent. Organophosphonic acids (PAs) were applied as surface modifiers to introduce Brønsted acid sites; various tail functionalities of the PA-modifier were applied to tune the Brønsted acidity. Reaction studies indicated that PA-modification significantly enhanced the HDO activity due to creation of the metal-acid bifunctionality. This improvement positively correlated to the Brønsted acidity introduced by the PA-modification.
After obtaining the knowledge of PA-promotion for vanillin HDO on Pd/Al2O3, the chemistry was further investigated in a biphasic water/oil Pickering emulsion system to better resemble the phase of bio-oil. PA-modifiers were again applied as surface modifiers. In addition to functioning as a catalytic promoter, PA-modification was aimed to tune the surface hydrophobicity by varying the tail length and functionality. Reaction studies and statistical analysis suggested a hydrophobic/hydrophilic effect on the HDO activity upon PA-modification. Kinetic fitting of the reaction profile further suggested that this effect was due to its influence on the selectivity to different reaction pathways.
Hao, Pengxiao, "Modifying Supported Palladium Catalysts Using Self-Assembled Monolayers for Liquid-Phase Reactions" (2018). Chemical & Biological Engineering Graduate Theses & Dissertations. 131.