Catalytic Transformation of Biomass-Derived Oxygenates Using Transition Metal Carbide, Nitride, and Oxide Surfaces

The catalytic conversion of biomass-derived oxygenates into valuable fuels and chemicals is a promising route to address the current energy and environmental issues. The low-temperature catalytic conversion is particularly promising in that it does not require intense energy input and can yield a variety of value-added products. In this approach, the hydrodeoxygenation reaction removes excess oxygen in biomass-derived oxygenates to convert them into value-added fuels and chemicals. There are two promising conversion pathways under this category: the conversion of lignocellulosic biomass and the biodiesel production from waste cooking oils. In the first approach, furfural is an important platform chemical that can be further upgraded into value-added products. Transition metal carbide catalysts have been demonstrated to be highly active and selective in cleaving the aliphatic C-O bond to form 2-methylfuran from furfural. However, the stability of these catalysts needs further improvement. In the second approach of biodiesel production, glycerol is a major by-product. The upgrading of glycerol via the selective hydrodeoxygenation reaction is especially economically promising. Different numbers of C-O bonds of glycerol can be cleaved to form value-added compounds, such as allyl alcohol, propanal, and acetol. Mo₂C has previously been shown as a selective catalyst for C-O bond scission, but its interaction with oxygen is so strong that all the C-O bonds in glycerol are cleaved. In order to selectively break certain numbers of C-O bonds while preserving others, the Mo2C catalyst needs to be modified. In this dissertation, the strategies for modifying the Mo₂C surface to achieve enhanced stability for furfural conversion and tunable selectivity for glycerol upgrading are demonstrated. With the addition of cobalt, the interaction between the surface and the oxygen atom in furfural is lowered to a proper extent and therefore the stability of Mo₂C is enhanced. By using different coverages of copper to modify Mo₂C, the number of C-O bonds cleaved in glycerol can be controlled. The subsequent chapters then compare the reactions of glycerol over the corresponding transition metal nitride, Mo₂N, as well as the C-O bond scission over a modified transition metal oxide, WOx/Pt(111), surface. It is shown that while Mo₂C and Mo₂N both break all C-O bonds of glycerol to produce propylene, Mo₂N also selectively cleaves two C-O bonds of glycerol to form allyl alcohol and propanal, a phenomenon only observed on the Cu/Mo₂C interface. DFT calculations reveal that the surface nitrogen atoms in Mo₂N block some Mo sites, and therefore promote the selective C-O bond scission. In the case of C-O bond scission of isopropanol over WOₓ/Pt(111), it is shown that surface hydroxyl groups on WOx sites catalyze the reaction. DFT results also demonstrate the synergistic effect between WOx and Pt and predict the energetics of in situ acid sites formation, which are very useful for the optimization of relevant metal oxide/metal catalysts. Overall, this dissertation compares the similarities and differences regarding the active sites and reaction mechanisms of C-O/C=O bond scission of biomass-derived oxygenates over transition metal carbide, nitride, and oxide surfaces, which should in turn provide useful guidelines for the rational catalyst design for biomass upgrading reactions