UC Santa Barbara

A variety of technical, economic, and societal factors require a global shift in the consumption of goods and the production of energy towards routes that are broadly more sustainable. A shift toward the use of biomass as a feedstock for carbon-based chemicals is one promising component of such a future shift, spurred on recently by rapid electrification of transportation and energy sectors. Lignin, a component of biomass that is currently underutilized as waste generated during cellulose production, represents a promising, drop-in replacement for current carbon feedstocks such as petroleum for the production of aromatic monomers such as benzene, toluene, and xylene (BTX). This route of lignin-to-chemicals would, in principle, provide a net-zero carbon emissions source of these high-value chemicals currently produced on the scale of hundreds of millions of tons annually across the globe. However, one of the primary technical challenges preventing the adoption of such a process is the removal of oxygen from lignin-derived monomers through catalytic hydrodeoxygenation (HDO). Oxygenated lignin monomers provide a range of reactive functionalities that are activated by the catalysts typically explored to date, namely supported transition metal nanoparticles. Thus, high selectivity of catalyst activation towards solely C-O bonds is essential to avoid off-target product formation. Recently, a class of catalysts involving noble metal-reducible metal oxide interactions has been identified as promising for the HDO of lignin-derived compounds. Despite initial reports of high selectivity for C-O bond cleavage over other competing reactions such as aromatic ring hydrogenation, there remains a lack of fundamental understanding of the active site, hampered primarily by vast uncertainty regarding the mechanism of deoxygenation, the structure of the catalysts under reaction conditions, and the subsequent structure-activity relationships. While it is generally known that the reduced oxide decorates the metal nanoparticle surfaces, it is unclear whether or not the oxide participates directly in catalysis either as the active site itself, a modifier to the base metal reactivity, or solely as a spectator or inhibitor. Because of this uncertainty, a number of plausible mechanisms have been proposed, including direct deoxygenation, acid-catalyzed dehydration, and Mars-van Krevelen-type deoxygenation, among others. This work contained within this dissertation seeks to clarify the role of the reducible metal oxide as it pertains to HDO catalysis, specifically, but also to expound upon the understanding of metal-oxide interactions more broadly, especially concerning the so-called strong metal-support interaction (SMSI). The development of a model catalyst system with high tunability which facilitates a number of different characterization techniques to probe catalyst structure and develop structure-activity relationship is critical to this goal. Through this developed catalyst framework, this dissertation explores the effects on HDO catalysis of nanoparticle size (Chapter 2), the metal oxide surface density (Chapter 3), metal oxide identity (Chapter 4), and support identity (Chapter 5).Through a combination of in situ and ex-situ characterization techniques (DRIFTS, TEM, chemisorption) and measured reactivity of diverse model compounds for lignin, the role of MOx promotion of Pt in HDO is clarified. MOx decoration on the surfaces of metal sites serves primarily as a site-blocker, selectively inhibiting the undesired aromatic hydrogenation by limiting the number of metal ensembles large enough to facilitate planar aromatic adsorption through the preferential decoration of these ensembles. However, the oxide-metal interface formed during catalysis also plays a lesser role in facilitating deoxygenation through the conversion of metastable surface intermediates. The results suggest that both modified and unmodified Pt catalysts transit through a shared mechanism in the HDO of phenolics, with tautomerization of the phenolic functionality being an essential characteristic to facilitate deoxygenation under the conditions studied.