Date of Award
Doctor of Philosophy (PhD)
Daniel K. Schwartz
Catalysis, the study of catalysts, is one of the most impactful, yet hidden sciences in society. For example, most are unaware that an estimated forty percent of people on earth would not be alive today without the discovery of certain catalysts. However, the understanding of catalytic properties and the means to design catalysts, de novo, elude modern catalysis research. Tools are being developed to control the near-surface environment and guide catalytic selectivity and activity. One such tool is self assembled monolayers (SAMs). Thiolate-based SAMs have been utilized with noble metal catalysts with great success, yet the approach has yet to be extended to other systems, such as metal oxides. This thesis intends to do just that, extend SAM strategies to control surface properties of metal oxides.
First, trimethylsilyl (TMS) was utilized on a traditional dehydration catalyst, γ-Al2O3, in a strategy called active site selection. The catalytic dehydration rate of 1,2-propanediol was increased by nearly 50% after functionalization with TMS. Surprisingly, traditional methods for measuring reaction site density demonstrated the functionalized catalyst had 60% the number of sites of the native. This was reflected in a decrease in the dehydration activity of 1-propanol and 2-propanol. Temperature programmed desorption of 1,2-propanediol demonstrated that TMS functionalized γ-Al2O3 caused a weakening in the binding strength. This suggested TMS prevented a non-reactive strong binding orientation, thus, upon functionalization, the reactant can enter a reactive conformation more easily allowing for higher rates of dehydration.
Reactions of simple alcohols on TiO2 are complex, resulting in dehydration, dehydrogenation and condensation. Several studies have demonstrated a coverage dependent selectivity; depending on high or low reactant coverage, dehydration or dehydrogenation is favored. We functionalized anatase TiO2 with phosphonic acid SAMs, thus creating artificially high coverage, resulting in high selectivity toward the dehydration product. Surprisingly, when the electronic properties, specifically the dipole moment, of the tail moiety were varied, the dehydration rate was also be varied. Through a series of experiments and computational efforts we demonstrated that aromatic phosphonic acid SAMs of varying dipole moments induce a near-surface electronic field. This change in the near-surface electronics caused a shift in the transition state geometry of dehydration resulting in higher dehydration rates.
Interestingly, high selectivity to dehydration over dehydrogenation for simple alcohols reacted over anatase TiO2 is independent of the structure of the SAM tail moiety. Phosphonic acids are able to bind to a variety of metal oxides. We explored how phosphonic acid SAMs changed the dehydration and dehydrogenation rate of simple alcohols on many metal oxides including: γ-Al2O3, CuO, CeO2, γ-Fe2O3, MgO, TiO2-anatase, TiO2-rutile, SnO2, WO3, ZnO, and ZrO2. In all cases, except CeO2, SnO2 and TiO2-anatase, we found significant decreases in dehydration and dehydrogenation activity. For the three exceptions, CeO2, SnO2, and TiO2-anatase phase, dehydration activity increased by up to ten times the native performance, while lowering dehydrogenation activity. I explore the bulk properties of these oxides as a means to explain this effect.
Thiolate SAMs have been utilized to tune the near-surface environment of supported noble-metal catalysts. Though this approach can significantly increase reaction selectivity, one significant drawback is lower reaction rates. The alumina support for these noble metals can be functionalized with phosphonic acid SAMs to increase the hy
Ellis, Lucas Delano, "Controlling Reaction and Adsorption on Metal Oxides with Self-Assembled Monolayers" (2019). Chemical & Biological Engineering Graduate Theses & Dissertations. 134.
Available for download on Thursday, January 27, 2022