Kinetic modeling and mechanisms for catalytic upgrade of biomass derivatives
Christiansen, Matthew A.
University of Delaware
The utilization of biomass as a renewable feedstock for commodity chemicals may be greatly benefitted by the successful development and application of heterogeneous catalysts for selective deoxygenation. This dissertation combines density functional theory (DFT) and microkinetic modeling to describe the kinetics and mechanisms of converting oxygenated hydrocarbons to commodity chemicals. The results of these studies provide fundamental descriptions of hydrogenation and dehydration, essential component chemistries of the overall hydrodeoxygenation (HDO) processes that lead to valuable chemical products. HDO catalysts typically possess multiple catalytic active sites, including a metallic site and an acidic functionality. A clear understanding of the individual capabilities of each site is achieved by first studying them in isolation. H2 generation and management are essential aspects of HDO processes, with generation kinetics thought to be primarily controlled by the metal sites. Those kinetics were explored via microkinetic modeling, using ethylene glycol as a bio-derived hydrogen donor molecule with a Pt catalyst. The H 2 formation rates from steam reforming of ethylene glycol are found to be well-described by a Pt-based mechanism, confirming the role of the metal sites in promoting this chemistry. Initial/early dehydrogenation of ethylene glycol controls the overall reaction rate, while water facilitates the downstream conversion of carbon monoxide into carbon dioxide without affecting the upstream (de)hydrogenation rates. γ-Al2 O3 is a well-known acidic heterogeneous catalyst support that has been used in HDO processes. Using DFT, the adsorption of several oxygenate probe molecules were explored on various crystallographic facets to understand how the binding strength is influenced by (1) the identity of the acid site, and (2) surface hydration. Further, ethanol in particular was selected to examine the mechanisms of dehydration and etherification on this material, in order to understand how the surface acidity promotes these reactions. Exposed Al sites exhibit Lewis acidity, while partial hydration of the surface creates possible Brønsted acid sites. The stability of ethanol adsorbed directly on Al sites was found to be superior to adsorption on a Brønsted-like site. The energetically preferred pathways for dehydration and etherification are concerted Lewis-catalyzed mechanisms, namely E2 mechanisms for dehydration and S N 2 mechanisms for etherification. The strength of adsorption and the magnitude of the reaction barriers may be strongly affected by the character of the Al site and the presence of co-adsorbed water. These effects are qualitatively and (in certain cases) quantitatively captured by a descriptor derived from the calculated electronic states of the γ-Al 2 O3 surface. In addition, kinetic dependencies identified through these calculations rationalize experimental selectivity trends to ethylene and diethyl ether. The DFT results were subsequently used to parameterize a multi-site (Al and O) mean-field microkinetic model for ethanol dehydration and etherification. Trends in experimental reaction orders were captured successfully by both the full model and analytical reduced rate expressions, and the E2 and S N 2 mechanisms are the rate-controlling steps in the network. This demonstrates the applicability of the DFT mechanisms to powdered γ-Al2 O 3 catalysts and makes a promising case for using multi-site mean-field models to understand acid-catalyzed metal oxide chemistries.